That is one of the continuity problems. He has spoken before (though apparently he doesn't like speaking & only does so during very emotional moments), and they have shown that he does understand when someone speaks to him or the conversations based on some of the face expressions they have given him to his reaction to what is said, and he has responded to questions asked him also before in some movies and tv episodes. Though, like I said, if one assumes this episode is happening on another earth like lbt earth 4, then on that world maybe he can't speak, nor understand much of what is said.