Opus Fast Mode is 30$/150$/M Input/Output cost. Mythos's pricing (from model card) is 25$/125$ Input/Output cost.
Based on this I doubt that Mythos pro is too dangerous to release or provides significantly more value.
You just disingenuously compared it to fast mode which is expensive not due to model strength or size but because you lose all kind of optimizations
As far as my understanding goes. It is not a breakthrough model itself but finetuned model with right tools and skills. Fairly similiar to today's coding agents with difference that they are made for software engineering not cyber security.