It's federated with Lemmy, yea, but it has a completely different layout with different features. I can straight up browse and interact with Mastodon from /kbin thanks to its microblog support, but you obviously wouldn't say /kbin is included in Mastodon. I think that the same goes for Lemmy.
Also, /kbin doesn't get a mention but beehaw (a Lemmy instance) does?