Unless I'm missing something, this isn't a messaging standard like RCS or XMPP. This is just a way to do E2EE messaging. Just like Signal and Whatsapp both use the Signal protocol for E2EE messaging but are completely siloed networks, MLS would be much the same.
While the RFC specifies a whole bunch on the payload structure there's more than enough in there left to the implementation. It will absolutely not allow interoperability unless messengers will specifically for on adding it. No more than WhatsApp and Skype are, even though both use the signal protocol.
I've been waiting for something like this again. Imagine being able to message your friends on Instagram from something like mastodon or lemmy, or vice versa (even though meta would never do that)