I think the wording here is kind of odd.

An unconstrained search will not find a siren world, or even a very good world. There are simply too many to consider. The problem is that you're likely to design an AI that finds worlds that you'd like. It may or may not actually show you anything, but you program it to give you what it thinks you'd rate the best. You're essentially programming it to design a siren world. It won't intentionally hide anything dark under there, but it will spend way too much effort on things that make the world look good. It might even end up with dark things hidden, just because they were somehow necessary to make it look that good.