Two of our children with autism are considered nonverbal. If you met our son, you would probably consider him low functioning. He doesn’t talk (much). He hums and moans and makes weird sounds. We have heard a number of times of people who should know better who think that he is unintelligent.
It just isn’t true.
Our son is very intelligent. He taught himself a song on the piano without a lesson or even the encouragement to play. He can read, in fact when we play a CD, he pulls out the lyrics to read along. Put him in front of a computer and he can navigate the internet no problem.
There has been some research done in this area. You can find the article here. The main point is that we should not be making assumptions about people with autism or anyone else.