I'm not sure about that entirely. It seems like kbin can federate with both mastodon and lemmy. Lemmy and kbin can federate with each other. But I don't think Lemmy and mastodon can right now. I don't know if that's deliberate or just unimplemented. I will say the kbin/mastodon interactions just seem... strange. Like you see mastodon users but I don't know why I wouldn't just use a mastodon client. But I haven't spent a lot of time directly inside kbin. I prefer the lemmy interface so far. People have described it as lemmy being like old.reddit and kbin being like new.reddit in feel.