Then you don't know what is, in my humble opinion.
Are you seriously suggesting that the advice to not let the adventurers find what you haven't created is railroading?
I think this is open to some legitimate debate. Saying "you can't find X no matter what you do" is certainly a railroading technique.
But given the full context of how Gygax designed his campaigns (with dungeons that constantly shifted and changed when you weren't looking at them), this particular application looks more like "they can't find it because it doesn't exist yet; if some sort of explanation is later demanded, here's a retcon you can use".
And failing to find something because it doesn't exist wouldn't be railroading.
So, it comes to page 12 of a conversation, and still folks are saying to each other that they don't know what the subject even is?
Classic.
Gray areas are gray; people disagree about them. Film at 11.