The fediverse is an umbrella term. I don’t know that anyone thinks of it as limited to activitypub.

Matrix might not interact with activitypub platforms, but it’s definitely federated, and definitely contains a universe of content.

I’ve found a couple active “spaces” (the equivalent of discord servers, a collection of users and rooms, not just one group chat) that are great.

I access all that from my own instance, thanks to federation.