Kbin is not lemmy, but it's very similar and almost fully compatible.
lemmy.world users can post to sh.itjust.works and vice verse, that's probably what you're seeing.
The front page is /all by default. You can change this in setting. You're probably seeing something in /all from another instance. You can also turn off federation in the sidebar if you're interesting in ONLY seeing kbin stuff until you turn it off.