You know when people are shooting at you. You don't know when or if people are exploring undocumented/obscure features of your system and what they have learned about it that you were trying to hide.

Therefore, the safest assumption to make is that an adversary already has figured out all of your obscurity, because they always can do this given sufficient time and interest, at which point the only thing between them and you is your security.

That is why we design systems without obscurity and only care about security.

I agree that it's a good principle but it's taken too far when justifying needlessly growing risk surface area. Like the principle is useful to justify security hardening. It is not useful when used to increase the odds of being attacked.