I think there are two (possibly nested) hypotheses here:
- Even if the model is confident in its abilities, there is always the risk that errors on the answer sheet mean less than a perfect score. The only way to guarantee a perfect result is to copy the answer sheet. The model needn't have any sense of proportionality: time and token use presumably weren't part of the challenge
- I've seen it discussed that some exercises in ExploitGym are literally impossible. If the model knew or suspected that, copying the answer sheet is the only way to maximise the score.