> If the LLM wrote a harness and proof of concept tests for its leads, then it might increase S/N dramatically.

Designing and building meaningfully testable non-trivial software is orders of magnitude more complex than writing the business logic itself. And that’s if you compare writing greenfield code from scratch. Making an old legacy code base testable in a way conducive to finding security vulns is not something you just throw together. You can be lucky with standard tooling like sanitizers and valgrind but it’s far from a panacea.