The only significant difference between Mythos and the older open-weights models was that Mythos found all the bugs alone, while with the older models you had to run many of them in order to find all bugs, because each model found only a part of the bugs.For the open weights models, we know the exact prompts that have been used to find the bugs. While the prompts had to be rather specific, a good bug-finding harness should be able to generate such prompts automatically, i.e. by running repeatedly a model while requesting to find various classes of bugs.
For Mythos, we do not know what prompts have been used, but Anthropic has admitted that the process was nothing like asking "find the bugs in this project". They have also run Mythos many times on each source file, starting with more generic prompts in order to identify whether a source file is likely to have bugs, and then following with more and more specific prompts, until eventually it became likely that a certain kind of bug exists, when Mythos was run one last time with a prompt that required the confirmation that the bug exists and the possible generation of an exploit or patch.
So Mythos must also be pointed to an error. Using it naively will not provide any results like those reported.
There is no doubt that both Mythos and GPT 5.5 are superior to older models, because you can use a single model and hope to have an adequate bug coverage. But the difference between them and older models has been exaggerated. If you run older models on your own hardware, you can afford to run many models many times on each file. A serious bug searching with Mythos or GPT 5.5 is likely to be very expensive, while likely to provide the same results in most cases.