Right, it's not that there is something actively pushing hexbear to them, but because there are fewer places for them to go, it becomes more likely that they will end up in the remaining instances.
For example - let's say 2 hexbear users like to look at memes. They were previously using the lemmy world meme community, but now, since they are defederated from there, they will go to different instances' meme communities. But then, more defererate, and it funnels a higher concentration of them into the remaining instances' meme communities.
This is the lemmy.ml memes community, so unless I am very mistaken about how things work, I don't think you found your way here from hexbear all.
If lemmy.ml defederated from hexbear, you would no longer see this community on the "fediverse all" section, just like you cannot see communities from lemmy.world.
The point being, as available communities become fewer and fewer, hexbear users looking for meme communities will probably concentrate on the few remaining to them, instead of being spread out over the many otherwise available.
Right I get that. Lemmy.ml is visible to you in your federated feed. It sounded like the person I responded to said that this was part of the hexbear feed, which I don't think it would be, since that only displays hexbear communities.
All beside the point though - the point is that as more and more instances defederate from hexbear, the pool of meme communities dwindles, funneling more hexbear people into the few remaining to them simply by the lack of other options.