D.P.
Learning to sign is equivalent to learning a new/2nd language. Myths regarding either impeding a child's ability to learn a primary language is not just unfounded but also discredited. In fact it can only benefit regardless of whether the child is autistic or not.
I have a 4 yo autistic child and she was taught to sign while she was in early on , at around 17 mos. The signs came easier than the words but did not stop her from attempting the verbal words. It lead to less frustrations and her being more open to learn. Now I'm happy to say she is very verbal.