These aren’t very good words to show that they can speak. Mama and papa are just lip based, they don’t involve the tongue at all, and don’t require varying tonality from vocal cords. Even cats can make sounds that sound like speech.

Also this provides no evidence that he understands language just because he says a word. Parrots can speak much better than this but they do not understand language. I think the fact that they can learn sign language is much more significant.