It's not an EU requirement (at the moment, at least). Interoperability is required, and MLS is an IETF standard (RFC 9420) to accomplish secure cross service messaging.

Several Facebook people (@fb.com) were taking part in protocol design, so I wouldn't say they'll never adopt it. It all depends on how much shit the EU is willing to take when it comes to interoperability.

MLS without MIMI isn't going to change much, though, and MIMI is still a work in progress.