Isn't the just based on a matrix server connecting to all your accounts?

What the EU forces them to do is to be able to send messages across service borders, so you could communicate with someone on WhatsApp, for example without having an account there yourself.

I do share the most upvoted comment's skepticism though - Meta and Apple will fight this tooth and nails and make it so cumbersome (and opt-in, of course), it will have no relevance in practice.

As a Signal user I'm also not happy that the very least I have to share with Meta is my phone number (that's also criticism towards Signal though, I guess).