It’s not just limited to Lemmy and kbin. Technically any Fediverse-compatible site can interact with them (ie. Mastodon, Pixelfed, Calckey, etc). It’s just nicest to do it from Lemmy or kbin because they are the most Reddit-like.

Think of them like email providers. If you have a gmail account you can send messages to yahoomail users without having a yahoomail account, even though they are wildly different email services. Same concept for all the lemmy and kbin servers.