#ActivityPub is super cool once you see it in action.
For instance, you can visit the new StarTrek lemmy server here: https://startrek.website/c/startrek. Looks like a reddit sub with posts, threaded comments, upvotes, &c.
OR you can follow the same server on Mastodon @startrek.
Every thread and comment shows up as a boosted post.
Cool, right? Now, say you find a comment that you want to reply to. Post through your favorite Mastodon app, and that feeds right back to the thread on the #Lemmy server! 🤯
They’re probably unrelated communities that happened to choose the same name, especially if it’s something generic like “politics” or whatever. Best to assume there’s no affiliation unless they specifically say otherwise.