Or is it basically a Lemmy-and-Mastodon singular hybrid instance?
Ummm... sure? At least kbin.social is a singular hybrid instance that has better-than-expected Lemmy-and-Mastodon compatibility. Not perfect yet, but pretty good.
Kbin is so new compared to Lemmy that there were only like 2 instances a few weeks ago and the second one wasn't in English lol. There are more now but kbin.social is still the biggest and the sentiment stuck around.
In a sense, kbin is different than lemmy, but they can access the same content on the "fediverse". It might be good to think about lemmy and kbin as different clients to access a decentralized web of content through a standard protocol. I can view stuff that originates on a lemmy instance from kbin, and I can view stuff that originates on a kbin instance from lemmy -- all because it's the same protocol.