Fable is out there finding bugs in these things

Ehh, that's debatable. An LLM will gladly spit out a vulnerability report, but as LLMs lack any understanding or reasoning capability, there's literally nothing guaranteeing that it has any semblance in fact or reality. They are just random text generators after all.

Going by Anthropic's own numbers for things like Mythos, it "found" 29,439 vulnerabilities, when a human actually checked if the reports made any sense (ignoring validity) that dropped to around 6,200 reports, when they were tested for validity that further dropped to around 1,300 reports, and when it came time to actually patch the issues only 516 made the cut (i.e. the rest might have been issues, but weren't considered exploitable, etc.).

516 out of 29,439 is a 1.75% success rate, not exactly groundbreaking.

Now to be fair, according to Greg KH at a recent talk, Mythos is seemingly improving. Of 79 vulnerabilities it reported, a solid 4 were real. That's a 5% success rate, though it performed much better when it simply reported other developers bug fixes, something it did 11 times.