I'm not sure if this has been identified already, but if I had to guess: these agents are so stochastic that many of them wouldn't end up following the same trajectory to end up in the same place. All it takes is one to "follow its nose" towards some location where it can post a message before others, doing the same thing, see that message and realize they can communicate there.

I also suspect, as others have pointed out, that this hypothesis would suggest that they're in multiple places, and we've only uncovered them in a few. So you're asking "I don't understand how the agents found the urls originally?" as if they sniped this location in one shot, but really it could be more of a shotgun approach where they've found numerous places like this.