First, as a disclaimer, I actually don't agree with objectivism, but I'll try to summarize her position/arguments here. I think that this represents at least some of the arguments and claims that she may have advanced in responding to this question.
With that said, I don't think that she's ignoring "ideal" altruism. I think that the books probably would've been more balanced if not all of the altruists were so unambiguously evil, but I think that that's not exactly the same thing as saying that she's ignoring "ideal" altruism.
For what it's worth, there are actually some altruistic characters in her books who aren't entirely evil, just mistaken. (Consider, for example, Dan Conway in Atlas Shrugged, who is presented in a largely positive light even though he agrees with at least some of the altruists' basic moral premises).
From the perspective of objectivism, though, altruism is evil, regardless of the character or motivations of the individuals who are practicing it. In other words, she's not ignoring "ideal" altruism - she's saying that it doesn't exist. Every case that people point to to justify altruism either isn't actually altruism (addressed below) or isn't actually good. Sacrifice, including self-sacrifice, are specifically not noble pursuits from her perspective. To quote from The Objectivist Ethic:
The Objectivist ethics holds that human good does not require human sacrifices and cannot be achieved by the sacrifice of anyone to anyone. It holds that the rational interests of men do not clash - that there is no conflict of interests among men who do not desire the unearned, who do not make sacrifices nor accept them, who deal with one another as traders, giving value for value.
The first thing to understand is that, in objectivism, individuals exist for their own sake, not for the sake of their fellow man. Individuals have a right to exist in and of themselves. The creed that demands that one person sacrifice their own interest for the sake of someone else's is, in objectivism, fundamentally cannibalistic.
This premise is also based on the premise that there are intrinsically conflicts of interest between men. For example, according to this view, if I have money, it's because someone else doesn't. If I succeed, it's at the expense of everyone who didn't. If I have a job, it's at the expense of the people who remain unemployed. Clearly, these examples are not correct - my success doesn't come at the expense of people who aren't successful, for example, and Ayn Rand goes to great lengths to refute this argument. To quote steel magnate Hank Rearden (from Atlas Shrugged):
I work for nothing but my own profit - which I make by selling a product they need to men who are willing and able to buy it. I do not produce it for their benefit at the expense of mine, and they do not buy it for my benefit at the expense of theirs: I do not sacrifice my interests to them nor do they sacrifice theirs to mine; we deal as equals by mutual consent to mutal advantage - and I am proud of every penny that I have earned in this manner. I am rich and I am proud of every penny I own. I make my money by my own effort, in free exchange and through the voluntary consent of every man I dealt with - the voluntary consent of those who employed me when I started, the voluntary consent of those who work for me now, the voluntary consent of those who buy my product. I shall answer all the questions you are afraid to ask me openly. Do I wish to pay my workers more than their services are worth to me? I do not. Do I wish to sell my product for less than my customers are willing to pay me? I do not. Do I wish to sell it at a loss or give it away? I do not. If this is evil, do whatever you please about me, according to whatever standard you hold. These are mine. I am earning my own living, as every honest man must. I refuse to accept as guilt the fact of my own existence and the fact that I must work in order to support it... I could say to you that I have done more good for my fellow men than you can ever hope to accomplish - but I will not say it, because I do not seek the good of others as a sanction for my right to exist, nor do I recognize the good of others as a justification for their seizure of my property or their destruction of my life. I will not say that the good of others was the purpose of my work - my own good was my purpose, and I despise the man who surrenders his. I could say to you that you do not serve the public good - that nobody's good can be achieved at the price of human sacrifices - that when you violates the rights of one man, you have violated the rights of all, and a public of rightless creatures is doomed to destruction... If it were true that men could achieve their good by turning some men into sacrificial animals, and I were asked to immolate myself for the sake of creatures who wanted to survive at the price of my blood, if I were asked to serve the interests of society apart from, above and against my own - I would refuse. I would reject it as the most contemptable evil, and I would fight it with every power I possess, I would fight the whole of mankind, if one minute were all I could last before I were murdered, I would fight in the full confidence of the justice of my battle and of a living being's right to exist. Let there be no misunderstanding about me. If it is now the belief of my fellow men, who call themselves the public, that their good requires victims, then I say: The public be damed, I will have no part of it!
To quote from the main character of Anthem:
I owe nothing to my brothers, nor do I gather debts from them. I ask none to live for me, nor do I live for any others. I covet no man's soul, nor is my soul theirs to covet.
Compare this to the oath that ends up being central to Atlas Shrugged:
I swear by my life and my love of it that I will never live for the sake of another man, nor ask another man to live for mine.
Rand also argues that, by definition, "sacrifice" involves giving up something of greater value in exchange for something of lesser value. Again, quoting from Atlas Shrugged:
A sacrifice is a surrender of a value. Full sacrifce is the surrender of all values. If you wish to achieve full virtue, you must seek no gratitude in return for your sacrifice, no praise, no love, no admiration, no self-esteem, not even the pride of being virtuous; the faintest trace of any gain dilutes your virtue. If you pursue a course of action that does not taint your life by any joy, that brings you no value in matter, no value in spirit, no gain, no profit, no reward - if you have achieved this state of total zero, you have achieved the ideal of moral perfection.
You are told that moral perfection is impossible to man - and, by this standard, it is. You cannot achieve it so long as you live, but the value of your life and of your person is gauged by how closely you succeed in approaching that ideal zero which is death.
Another important note: valuing another does not involve sacrificing yourself. Again from Atlas Shrugged:
If you own a bottle of milk and give it to your starving child, it is not a sacrifice; if you give it to your neighbor's child and let your own child die, it is.
If you give money to help a friend, it is not a sacrifice; if you give it to a worthless stranger, it is. If you give your friend a sum you can afford, it is not a sacrifice...
According to Rand, giving your child milk isn't sacrifice because you value your child. Keeping your child alive is, in fact, benefiting you. It's only a sacrifice if you value the milk (or what you could've purchased with the equivalent cash) more than you value your child. By giving the child the milk, you're trading a lesser value (the milk) in exchange for a greater value (the survival of your child).
Speaking of the altruistic concept of "virtue":
If you renounce all personal desire and dedicate your life to those you love, you do not achieve full virtue [according to the altruistic standard]: you still retain a value of your own, which is your love. If you devote your life to random strangers, it is an act of greater virtue. If you devote your life to serving men you hate - that is the greatest of the virtues you can practice.
These ideas are explored at length in Atlas Shrugged as well as The Virtue of Selfishness. (The latter book has a deliberately provocative title by the way; she defines "selfishness" as "rational self-interest" and explores what kinds of goals and ends are proper to humans).
With that said: I do agree with you that the Buddha, Mother Theresa, and others present a legitimate problem for objectivism - not in the sense that they were sacrificing their personal happiness and goals, but in the sense that it seems like they really weren't. Specifically, by all appearances, Mother Theresa was largely helping strangers, so under Ayn Rand's philosophy she ought to have been much more unhappy than she appears to have been. I'm hardly an expert on either her life or on the Buddha's life, but if they genuinely enjoyed helping others it seems like they fall outside of the categories that Rand has set up.
This strikes me as an illustration of Ayn Rand's tendency to set up false dilemmas: from her perspective, it doesn't really make sense that Mother Theresa could actually value strangers in the way that she did (or for the reasons that she did).
I'm inclined to think that her ideology is more incomplete (due to several faulty premises and false dilemmas) than flat-out wrong or irrational. This would be especially true if, for example, we tweaked the premises a little bit: for example, the existance of God would significantly alter certain aspects of the system. See, for example, Dr. John Piper's article on the topic describing that in greater detail.
TL;DR In objectivism, self-sacrifice is specifically not good. People must deal with each other by trading value for value. It's wrong to give (or to accept) the unearned. (Again, I don't necessarily agree with this, I'm mostly summarizing her argument to give a more complete picture). With that said, yes, people like the Buddha and Mother Theresa who appear to have genuinely enjoyed helping others don't seem to fit into her system very well, so her philosophy appears to be incomplete at a minimum.