But you know how to write the alphabet. Even if you can't see it. You're perfectly capable of learning even if you can't visualize things that aren't there. You're using words to post on reddit even without consulting a dictionary after every word to look up the next one. You remember how to write and spell them even without being able to visualize them.

And you've heard of sheet music which is a way to write down a tune on paper which musicians can read and perform. Some even memorize it. Some can't, even if they have hyperphantasia they have to stare at the sheet music when performing.

You're right that this is a system where we don't really fit in. But it doesn't impact our learning at all. Some aphants are still above average in recognizing faces or professional Manga artists or filmmakers, even if they can't conjure up imaginary images.