Signal, no. And yes, Lemmy's integration via AP is sub-perfect. Ie, I can (and do) follow communities on lemmy, with my Mastodon and pixelfed accounts.

So, work is needed, and only happens if a) same protocol is used, or b) bridge modules are used (like friendica does).

If someone made an xmpp bridge to talk AP, then it's would be one big network, like how a lot of irc nets get bridged with xmpp nets, which makes those one, singular, federated network. But until they start speaking the protocol the rest of the fediverse does, it's just another network.

And again, it's not a bad thing. It'll even probably get there eventually.