Threads is leveraging Instagram to fast start a massive user base, and while it's still smaller than Twitter, a lot of people will bail on Twitter (and BlueSky, Mastodon, et al.) once they think everybody is on Threads instead. That's the kind of weight these mega-corps have to throw around.
Granted, Threads isn't in Europe because it would run afoul of privacy protections, but they may not care. They may not care about joining the Fediverse ultimately either. They could seriously fracture the Fediverse if they do, however, as people come down on either defederating from its massive user base or trying to glom onto it. Of course, Threads could also fizzle out as Instagram users get bored with it.