I will say I'm pretty sure I saw people theorize this statement was true fairly early on in the Games

I don't know if anyone could've predicted the exact form the Seventh Coil took, but I remember some discussion on how the vibes around the Sixth Coil were immediately weird, how the promises were too good to be true, how dreams of banded orange/black/white and fangs could refer to snakes as well as tigers, and thus drawing the conclusion that there must have been The Most Enormous Boil imprisoned down there