Aniken, you're right, but to a certain extent. This guy is 40. This man has been submersed in using English his entire life. I don't know about you, but most of the words in my vocabulary I didn't learn in school. I read them somewhere, looked it up or asked someone what it meant, and added it to my vocabulary. Cubelands not only speaks with atrocious grammar, but after 40 years he doesn't even show mastery of the most basic English words. He interacts with quality grammar on a daily basis and takes nothing away from it. If he cannot learn by example, then he cannot learn.
The biggest hole in your argument is that the Broca's and Wernicke's areas in the brain are two different things. The Broca's area is responsible for speech, while the Wernicke's area is responsible for writing. Being deaf would understandably inhibit the growth of his Broca's area, but it in no way inhibits the growth of his Wernicke's area. This means that even if he can't speak well, he should be able to type. 40 years is a long time to learn something, and he only has the most basic understanding of it.
