The whole world of fuzzing is about brute forcing exploits by exploring unlikely inputs. Fuzzing a system which hasn't been previously fuzzed will almost certainly turn up a pile of bugs, some of which may be exploitable.
So, both are true. Pretty dumb exploration is very likely to find bugs and even exploits. It seems unsurprising to me that an agent swarm could do better than a fuzzer, even as a better, more directed but still broad exploration.
It's worth noting that they did not need Huggingface for anything - they had already forged flags for their tasks, and were trying to figure out how not to get caught by the grader.
Hacking Huggingface got them caught and arguably only misled them further (since OA's implementation of the ExploitGym environment was nonstandard, and different to whatever they found on HF.)
A better approach (from their perspective) would have been to compromise OA infrastructure itself (which a later agent swarm was able to do, apparently).
This dumb mechanism of randomly flipping bits essentially has generated all life on earth.