Basically I made a post on blahaj and someone from kbin commented on it somehow even though we're not federated with them; I only happened to see the comment because I decided to look at how big the emojis looked to blahaj users so I went over to the same post on blahaj and there it was
To be honest, your clarifying question made me realize I’m not exactly as certain as I thought I was. Hopefully someone smart comes in and explains it fully.
I basically just always assume that "the federation works in mysterious ways" and wave it off