Setting a swarm of agents loose for hours to look for software vulnerabilities is far more compute-expensive than fuzzing. The industry has never thrown this kind of compute resources at pure fuzzing, in part because you can't get much VC money for that.
This sounds like a swarm of agents with particular prompting that happens to guide the LLMs toward doing a lot of fuzzing, so it's not either/or; you're getting all the compute requirements of both.
You also quickly get into diminishing returns with fuzzing. Generally a bug is either going to be found relatively quickly with a given fuzzing approach or it's going to be nearly impossible to find. You're usually better coming up with more intelligent fuzzing approaches than you are just dumping compute into it.