There was a Mastodon thread by someone who had decompiled the leaked Android APK for Threads the other day. There were a few strings in there talking about the fediverse and federation with other servers, so it seems like they at least planned to add it at some point.
Well, it was announced that naughty old mr car would be buying twitter in about April last year. From then, there was enough chance it might all go horribly wrong that maybe they could justify this project.
Could it be that this was put in together in a couple sprints as they saw Twitter in a weak position?
Or maybe they have a lot of products like this on the bench and release them when the time is right?