Whilst I agree that they shouldn't, it's a fact that Threads exists and, as far as I know, is not part of the Threadiverse but is in an adjacent space. To the point where I personally, someone involved in the Fediverse, am not sure whether it communicates with the "Threadiverse". Mastodon does, for example.
If we were talking about a family of phones called "Threads" then that wouldn't be all that confusing, but when it's another social app then it's a mess.