> Messaging applications are increasingly making use of end-to-end security mechanisms to ensure that messages are only accessible to the communicating endpoints, and not to any servers involved in delivering messages. Establishing keys to provide such protections is challenging for group chat settings, in which more than two clients need to agree on a key but may not be online at the same time. In this document, we specify a key establishment protocol that provides efficient asynchronous group key establishment with forward secrecy (FS) and post-compromise security (PCS) for groups in size ranging from two to thousands.
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”.
(Much of this how-useful-is-it stuff is implied by the name Message Layer Security (MLS), by analogy to Transport Layer Security (TLS) and the functionality of layers inside it like HTTP/1.1.)