Meta (when they were Facebook) had a rule that members would use their real names, that their users consented to as part of the terms of service. Apparently they used that as justification to find out someone's real name who was using a pseudonym. Kinda stinks, but I could see how they could justify it (though in the cases I'm aware of they just demanded that people provide real names or verify that the name that they were using was real).

But someone who uses a completely different site hasn't consented to the Threads terms of service, and if Threads randomly decided to dox people and alter their posts to add real names (or perhaps deadnames for trans folks), that's a very different matter, and I'm sure that their lawyers are going to tell them not to do that (or risk legal consequences). Since non-users haven't agreed to any terms of service Meta face real court, none of that binding arbitration stuff, and possibly class action if they do it a lot.