Maybe this is just Anthropic pre-IPO marketing to try to convince people how much better Mythos is than Opus 4.8. There sure seemed to be a lot of shills out on release day talking about how it was a "step change" (exact phrase) in capability.
My impression is that Anthropic's point about Mythos is that it is uniquely good at finding vulnerabilities and then using them to create working exploit chains.
There is some meaningful evidence that Fable is fine-tuned or steered away from helping on this very task, which is not something that can be feasibly circumvented by a basic jailbreak.