>So, it’s just the end-to-end encryption stuff with double ratcheting and such, not anything about a particular message format. This makes it a very valuable element for interoperability, and I think might make bridging work better?; but it’s not sufficient for mixing messaging services in any way, nor is Google proposing anything like XMPP of “implement this and you can federate with our service”.

There's ongoing work at the IETF to create a new protocol called MIMI (More Instant Messaging Interoperability)[0], which will use MLS as its E2E crypto layer.

There are Google reps on the working group, amongst others.

[0] https://datatracker.ietf.org/group/mimi/about/