If there are multiple protocols, then that defeats the entire purpose of a protocol. If Matrix and ActivityPub are in the Fediverse, then Facebook and Twitter should be, too.
Facebook can’t talk with Twitter, so they aren’t federated with each other. Same goes for ActivityPub and Matrix. Fediverse doesn’t mean “has a federated protocol”. It means “these applications are federated with each other” (from what I understand).
Doesn’t Fediverse mean “these applications can federate with each other”? How would you federate with Lemmy without ActivityPub?