Meta says that it will only allow third-party developers to use another protocol besides Signal, “if they are able to demonstrate it offers the same security guarantees as Signal.”
If matrix finally finishes implementing MLS, maybe they could convince meta to use it.
Last time they touched an open chat protocol, they hung it out to dry. That was XMPP. That's why more than half of the fediverse is reluctant or outright hostile to federate with anything meta.
You are underplaying the damage Google and FB did to XMPP. It wasn't supposed to be relegated to an obscure backend protocol. The involvement of those companies ensured that it didn't become a popular user-facing protocol.