They are told outside the test because if they can't find it when given hints then it's safe to assume it won't find it given no hints. It verifies to test, to an extent, much like running tests that should fail when given a set of inputs that should make it fail (you write an always failing test alongside your other tests, right?;)

No, the purpose was to create a (automated) test set in the first place. The author builds an LLM judge which can score the LLMs participating during test-time. That would be why the author used the strongest model (Opus 4,7 at the time) as the judge.