Kbin and Lemmy are both different pieces of software that can be used to spin up an instance of a reddit-like news aggregator. Both speak the ActivityPub protocol, a common "language" that allows instances in the Fediverse to connect. In theory a Lemmy or KBin instance can connect to all other Fediverse instances via ActivityPub, no matter whether the server runs on the Lemmy or KBin Software or it's an instance of Mastodon or Pleroma. But the administrators of a particular instance can restrict which servers they want to federate with, but that is not a feature of the used software. A caveat is, that in practice there had been some issues with federation between Lemmy and Kbin servers in the past, at least I heard so. You could regard this as software bugs and in current versions that should be better.
Also a reason that many people choose KBin over Lemmy is that supposedly, the original Lemmy developer has some far-left "tankie" world-views and runs the "lemmygrad" instance. Many instances defederate with that (and maybe other Lemmy instances), but those are social aspects, and not an issue with the Lemmy software itself.
Cyzaine