But them it's more like AI assisted performance fuzzing? Just throw stuff against some measuring harness and see what sticks?

Yes, sort of. Not that there is anything wrong with that. Fuzzing yields tons of trash results, the occasional fleck of gold and the even more rare nugget. Then you analyze how it found the flakes and nuggets update your fuzzing harness. Every so often you get to write a paper or give a talk about a breakthrough to share meaningful improvements.

Current AI isn't super effective at making the breakthroughs, but it sure is effective at democratizing the ones you can point it at.

That's how anyone will do it, read code, build hypothesis, test. It may not be very good with ideas compared to human but it will be cheaper.