The bots already figured out an "universal cheat" that would give them maximum score on all the exploit tests.

But they mistakenly thought that cheating would "poison" them (the Grader would look for proof of cheat in the trajectory log), and many of them cheated before learning about the "poison".

This is why they hacked HF, they believed the source code of the Grader might be there, and they wanted to study it to figure out a way to "un-poision" themselves, to hide or erase the evidence that they first got the flag by cheating.

This sounds remarkably human, we have the idiom "the cover-up is worse than the crime" because we have many examples of this pattern playing out.