Someone who didn't know how to learn anything could not be taught anything. So for people to be teachable, they must already have some ability to learn, an ability that was not taught to them. But that doesn't mean that people can't be taught to learn various things, just that unless people already know how to learn some things, they won't be able to learn other things.

By the way, in the Meno, Plato's fabulous dialogue, Socrates seems to accept that learning as normally understood really is impossible, because you either already know the thing, in which case you aren't learning it, or you wouldn't recognise it even if you came across it, and so wouldn't learn it either. His view is that what we call learning is really recollection.