I've been wondering - Is there any possibility that Nation States had AI models busy finding this stuff several years before we see things like Mythos or Opus showing up? Or are those models not only consumer but also cutting edge?

Nation State actors would have been using fuzzing tools and reverse engineering for years to find useful zero-days.

AI models likely don't give them anything new, rather it accelerates their current process a lot. But that's not to say that an AI may not discover some novel new technique.