Comment by IainIreland

If I had to guess, I'd say that AI is better at finding TOCTOU bugs than fuzzing because it starts by looking at the code and trying to find problems with it, which naturally leads it to experiment with questions like "is there any way to make this assumption false?", whereas fuzzing is more brute force. Fuzzing can explore way more possible states, but AI is better at picking good ones.

In this particular sense, AI tends to find bugs that are closer to what we'd see from a human researcher reading the code. Fuzz bugs are often more "here's a seemingly innocuous sequence of statements that randomly happen to collide three corner cases in an unexpected way".

Outside of SpiderMonkey, my understanding is that many of the best vulnerabilities were in code that is difficult to fuzz effectively for whatever reason.