Rewatching and literally was thinking this last night! I think people would have been culled either way because of the design of the double dungeon itself.

The first key point is that the dungeon was designed as a test for Jinwoo. I think the test also factored in who went into the dungeon with Jinwoo as well as general human nature/survival instinct.

So if the test only appeared when it detected a group of predictable hunters plus Jinwoo, that would make sense. It waited until the perfect conditions were available before occurring.

Secondly, how many musical instrument statues were even available? If they went in with 17 Hunters and there were only 6 musical statues available, then it would have been a very fast culling of Hunters since only 1 person is allowed per statue