It's not lemmy.
kbin began as a fork of Lemmy a while ago, but it's been overhauled into it's totally own thing.
It's interoperable with Lemmy, though. Like lemmy, it's a content aggregator, and kbin users can subscribe to lemmy communities (and Lemmy users can subscribe to kbin communities), but kbin communities (called "magazines") have some extra bells and whistles, most notably the ability to also bring in microblog posts that contain mod defined keywords or hashtags. This means you can also see what people are saying on Mastodon (or Calckey, or Misskey, or Akkoma, or...) about the topics the magazine focuses on.
There are a lot of interoperable website engines out here in the fediverse. Aside from kbin interoperability:
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Mastodon users can follow lemmy communities and comment on lemmy posts.
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Lemmy users can subscribe to PeerTube channels (check out, a random example I dug up, [email protected] or enter 'https://neat.tube/c/techlore/videos' into your instance's search bar to fetch it, if it's not available on your instance yet; that's a PeerTube channel, not a Lemmy community).
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Lemmy users can also view and interact with Frierndica groups (and vice versa; check out [email protected], which is a Friendica group).
Edit: See below.