The chain of trust being formed. If some adversary does slip past the radar, and gets guaranteed, once you revoke their access you're revoking the access of everyone else guaranteed by that person, by their guarantees, by their guarantees' guarantees, etc. recursively.

For example. Let's say that Alice is confirmed human (as you need to start somewhere, right?). Alice guarantees Bob and Charlie, saying "they're humans, let them in!". Bob is a good user and guarantees Dan and Ed. Now all five have access to the resource.

But let's say that Charlie is an adversary. She uses the system to guarantee a bunch of bots. And you detect bots in your network. They all backtrack to Charlie; so once you revoke access to Charlie, everyone else that she guaranteed loses access to the network. And their guarantees, etc. recursively.

If Charlie happened to also recruit a human, like Fran, Fran will also get orphaned like the bots. However Fran can simply ask someone else to be her guarantee.

[I'll edit this comment with a picture illustrating the process.]

EDIT: shitty infographic, behold!

Note that the Fediseer works in a simpler way, as each instance can only guarantee another instance (in this example I'm allowing multiple people to be guaranteed by the same person). However, the underlying reasoning is the same.