My understanding of flow is that it is triggered when the task is neither too easy or too hard. It's a sweet spot that maximizes your engagement with the task at hand.

Engagement != learning. If anything, I'd say engagement is closer corelated to output.

According to studies done on musical students (for what these studies are worth), the ones who practiced in "flow" progressed much slower than those who constantly and deliberately pushed themselves far outside their comfort zone.

Why can't so easy for you that you can lose yourself in it fall within neither too easy or too hard range?

Because it fails to emphasize that there is any difficulty at all.

I guess that's the point, to step back from the desire to always find something better. This somehow implies that you don't actively challenge yourself to learn new stuff. You could however learn new things nevertheless, but I guess the idea is not self-improvement, but getting at peace with yourself, and maybe also your limitations.

(Not advocating that, that's just how I would understand the concept)

Not at all. Flow is reached the moment I tackle a challenge that is just a bit above my skills. Then I learn. Learning something new does not have to be unpleasant. It is not easy to find the adequate challenge at each moment but it's possible and that's why some teachers, courses, books etc are much better than others.

Not necessarily true. If the task is too easy then you will get bored fairly soon.

I know many grandmas that don't ever seem to get bored knitting :)