That question is really the point. Viewed in the Heptapod fashion, there isn't really any meaning to the question of what it means to "always know" as distinct from "learning at some point".
The comparison to Lagrangian mechanics is helpful. Does the photon "know" that it's taking the shortest path to some point under the water? Or does it make a "decision", instant by instant, "learning" that it should turn when it enters the water surface?
The two are equivalent formulations, just viewed from different points of view. One point of view has all of history built in from the beginning; the other studies moment by moment.
It's not really an apt way to view humanity. The thing that makes a human brain can't avoid its progressive approach to history. Being bound to a "now" is fundamental to how your brain works. To be free of it, you'd have to stop being "you". Everything would change. It wouldn't give you the ability to predict the future; "predicting the future" would be something that wouldn't make any sense to you because you'd have redefined your self as something for which future and past are one.
It's probably not realistic to posit heptapods with brains that really work so much differently. (And if they were they'd be even less explicable.) But that's not really what the book is about. It's about grief and loss. How would you feel if you really could take the long term view: that every joyful birth was tempered with the grief of death, and vice versa? Can you get over loss by taking a different world-view?
I wasn't entirely happy with the way that was lost in the film, treating the time sense as a magic power that could predict the future. It was perhaps necessary to add an element like that in transferring it to the medium of film, but the story is much more meditative. As with all good sci-fi, it's not really about aliens; it's about ourselves.