Specifically, 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.”
The company blog post also explains that the solution, which builds on Meta’s existing client/server architecture, is the best, as it would lower the barriers for new entrants to participate. But this sets up Meta as the one making the rules and deciding how interop will work, of course. Meta notes that doing it this way will improve reliability, as Meta’s infrastructure has already been scaled to handle over 100 billion messages daily.