Lemmy and Kbin basically just use the same protocol for exchanging information.
Both are similar in that they interpret and present that information in a way that looks similar to Reddit which is why you can see a community on Lemmy as a magazine on kbin and vice versa. In addition Kbin also can interpret it in a way that resembles twitter.
They both started with the protocol but the way they store, serve, and present the information from the protocol is different. For example, I think Kbin stores the information that shows who upvoted what, but I don’t think Lemmy does.