There’s a double meaning in this line.
The stanza, considered on its own, describes various forms of persuasive argument, together with some satirical commentary. For example, line 4 (“For reason thinks all reasoning out of season”) seems paradoxical: how can ‘reason’ be opposed to ‘reasoning’? Byron is referring us to a particular philosophical sense of the word, current at the time:
reason, n. II. 5. c. A faculty transcending understanding, by which first principles are grasped a priori. Associated principally with Kant, in whose writings reason (German Vernunft) is opposed to understanding (German Verstand).
Oxford English Dictionary
This sense of the word was preferred by Coleridge, for example in his quatrain ‘Reason’, and Byron was fond of twitting Coleridge, for example:
Shall gentle Coleridge pass unnoticed here,
To turgid ode and tumid stanza dear?
Lord Byron (1810), English Bards, and Scotch Reviewers; a Satire, ll. 249–250.
So we suspect that in line 4 the poet is indulging in a little dig at Kant and Coleridge: their ‘Reason’ is quite opposed to ordinary logic and common sense.
In this satirical context, the last line is a complaint about all forms of rhetoric and philosophy, namely that no one keeps his argument short in length. This is emphasized by the use of ‘whine’ and ‘continue’ in the preceding lines.
But now consider the stanza in the context of the canto. Don Juan and John Johnson have been sold in the slave market of Constantinople and are being escorted by the eunuch Baba to the Sultan’s seraglio. Juan suggests that they attack their guide and escape:
“Let’s knock that old black fellow on the head,
“And march away–’twere easier done than said.”–
But Johnson is reluctant to acquiesce in this plan because at least as a slave he will get something to eat:
Besides, I’m hungry, and just now would take,
Like Esau, for my birthright a beefsteak.
(In the book of Genesis 25:29–34, Esau sells his birthright to his brother Jacob for a “mess of pottage”.)
At this point they pass the Sultan’s kitchens and Juan is distracted by the smells of cooking:
And nearer as they came, a genial Savour
Of certain stews, and roast-meats, and pilaus,
Things which in hungry mortals’ eyes find favour,
Made Juan in his harsh intentions pause,
In this wider context short suggests short of food, the point being that no form of reasoning or argument is as powerful as physical privation, as Byron goes on to explain in the following stanza:
But I digress: of all appeals,—although
I grant the power of pathos, and of gold,
Of beauty, flattery, threats, a shilling,—no
Method's more sure at moments to take hold
Of the best feelings of mankind, which grow
More tender, as we every day behold,
Than that all-softening, overpowering knell,
The tocsin of the soul—the dinner-bell.