>> I knew a loads of words from a several of authors and could grasp any grammatical construct easily. But I had a big problem with speaking or writing: I couldn't find words to express myself.

That happens, for sure, but we know that humans can also perform this task well. So that is maybe not such a good example? We know that humans can generalise learning very well, in general.

I mean, just looking at a single person's ability at a task doesn't help us understand the limits of human ability on that task.