Neither nostr nor simplex make “community building” a thing. The most important defining point is the ability to have communal intermediaries. All protocols can do a forwarding bot, starting with the good old mailing-list, but anything more complicated than that is rare. In nostr that would be running a full relay (which is just out of the question if you’re not technical). Simplex isn’t much better. Both are built for individualistic purposes, so it’s not really surprising.

ActivityPub isn’t perfect but it has Groups, with some people working on making them controllable. XMPP has highly configurable pubsub. Those are proper foundations for billions of people building billions of communities