What I'm saying is to let them have their own fediverse and keep them out of ours. Meta has already taken over Instagram and Whatsapp, and considered buying TikTok. Why should they have this too?

The fediverse exists as a shared commons currently; large, for-profit entities don't like shared alternatives they can't profit off as that is lost market share. From Meta's standpoint, they have to kill it or take it over and monetize it (and then kill it) as they can't maximize their profit from it otherwise. Coexistence with a competing paradigm (namely, a decenteralized, primarily non-profit social media competitor) is temporary and non of Meta's actions will go decreasing their profits except as a temporary loss leader to kill their competition (like Walmart going into a community and lowering their prices to kill their organic competition or Amazon killing bookstores). Again, Meta has already proven that they will take over competing social media and it would be foolish to think that they would not try again. If the fediverse were really not a threat to them, why would they create a product that ties into it and then ask the current fediverse operators to sign an NDA to discuss it. If the 12 million users (with 1.8 million active users, which, as a side note is a great demonstration of the pareto principal) didn't pose either a potential market or a up and coming threat to Meta, why would they bother? It's not like the fediverse could ever go viral and become a competitor like TikTok became to pre-existing social media.

Secondly I think EEE can also aim at “just” the featureset and technicalities

To think that Meta would not use their app to bias their massive user base to their own subs that they would then monetize, manipulate, and then cut off from the fediverse when they reach a critical mass is (respectfully) naive (as demonstrated by Google and XMPP or to a lesser extent with Apple iMessage and SMS/RCS). To think that Meta wouldn't add proprietary features that are not interoperable with the existing fediverse is also naive (see, Reddit closing their source, adding a bunch of features like chat functionality but not allowing 3rd party applications to use them).

If the subs with the most engagement are Meta operated, they will either degrade the experience of non-Meta users to the point where those users will switch to the Meta service, or just defederate as it will no longer be in their financial interest to federate with something that they have already subsumed. They will treat the existing fediverse like Reddit treated Apollo and the 3rd party application ecosystem. Do you really want that?