I’ve begun to wonder about this question. There’s nothing implied in the existence of the fediverse that it will include every site, and there’s nothing implied in the existence of every non-fediverse site which would stop them from forming their own fediverse. One could technically say email providers have already achieved this, though this only really applies to email providers which doesn’t make it as useful as the fediverse we know and love. Are there actually other fediverses out there?
I’m not an expert on this, but the fediverse is just a collection of things that can all communicate on a federated standard. It’s just like email, you can use whatever mail provider you want but you can email anyone else using email.
I guess your question is kind of like “are there other emails?” - and yeah, I guess there are other platforms where you can communicate and send messages, but they aren’t email.