I have been following the DMA closely, and so far it has been a big disappointment, just as I expected.
The way the EU approaches this walled garden problem, is to try and offer ways for other competitors to tap into the user base of the bigger players instead of trying to allow all EU citizens to chat with any other EU citizen who uses META Products regardless of their host platform. meaning "us" people who wish to self host an xmpp or Matrix servers and chat with facebook friends, It won't be straight forward or entirely possible for us to do so. unless maybe by doing a KYC with META. and signing up very stringent service agreements.
Meta will be creating all sorts of hurdles the DMA laws will allow them to, to cripple interoperability, from making other plateform signing up to special permissions from Meta, to hiding interoperability settings and making them opt-in, and building a scary rhetoric why you shouldn't be allowing other people outside of META to get in contact with you. There are some valid concerns, but I suspect Meta will implement the most spiteful procedures they can get away with, then spin up a rhetoric about proving their users being massing against interoperability.