Is it any service that includes the ability to federate? If so, are Matrix/XMPP/other non-ActivityPub services part of the Fediverse, or does Fediverse really just mean ActivityPub?
The context is wanting to promote a Matrix chat room on discuss.online: #online.discuss:discuss.online (see post). Does it belong in Lemmy communities like this one, !fedigrow@lemm.ee, etc? Or does it not count as part of the Fediverse?
Matrix servers can talk to each other, so yes, they are federated. But Matrix is not ActivityPub.