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.

A lemmy.world user posted in a sh.itjust.works community. Then it's propagated too all federated instances via a protocol called ActivityPub.

I think this is my take as well. It's probably worth it to post to the top two or so, if there isn't a clearly active one.

Eventually this might have to flip if we manage to grow big enough.

Could you explain this further, or post a link to the relevant documentation. I'm not sure how the tags system works but if it makes things more convenient it might be worth it to look into that.

Getting the same exact thing, including in the above thread.