Sort of. Every account on Lemmy federates with other Lemmy accounts using the activity pub protocol, but that protocol is vast and the way to implement it is left up to individual app developers. Some apps do tweet like posts very well but not reddit like posts at all. Some do both. Some do other weird stuff like bookwyrm where they use “ordered collections” to store book lists. So you can see Lemmy accounts on mastodon but it doesn’t work perfectly all the time.

I’m probably just parroting vague vibes like you say. The first time I heard of pleroma was in the context of Lemmy.world defederating from a ton of pleroma instances that were far right, or otherwise undesirable. But that’s more the users and not the software itself.

Oh man, I was going to use rebased since I heard the pleroma folks sucked. Guess I need to do more research.