That depends on the definition of mental retardation. We say mental retardation can be diagnosed by testing. Autistic people often test as retarded because of a basically dysfunctional movement system. These results are then verified by assumptions about how a person moves and talks when he is retarded. So when a person asserts that autism and mental retardation occur together, mention this supposedly retarded person who is a college student and say that you don't believe it.
Movement is essential to communication.
I am unable to speak because of apraxia, a movement disorder. I can type because the movement of one finger going up and down is much easier than speaking or using sign language. Even this simple movement is awfully hard. I began using facilitated communication in 1991 and needed support at my wrist. After five years of fading support I was able to type without the facilitator touching me.