Talking to yourself when you're alone isn't that much of a red flag really, especially for autistic people who often don't have a very well-developed inner monologue. For a lot of these cases, they will talk to themselves when they're alone to express their own internal thoughts. It's less "crazy hearing voices" and more "thinking out loud".
Talking to yourself in public is a much bigger red flag for a major mental illness, whether you're autistic or not. The issue of Chris talking to himself is whether or not he's babbling in public about the dimensional merge or if he's just expressing random thoughts out loud when he's alone in his room or in front of Barb, who's used to it already.