Yep. Federation could conceivably respond to the EU's requirement for interoperability -- and they could do it in a way that puts a lot of barriers to people actually moving, so works well for them. Of course the EU would say that didn't meet the requirement, which would lead to a multi-year legal battle and eventually Meta would probably pay a billion dollar fine (as they routinely do -- it's just a cost of doing business) and promise to remove the barriers (which they wouldn't, and then there would be another multi-year legal battle).
But none of that works if the EU won't allow Threads for some other reason!
Still, my guess is that they'll figure out a way around the EU's objections to Threads ... we shall see ...