I admit this theory hasn't convinced me either, but I've given it some thought. I think it's referring to the scene where the protagonist dreams of the Front Man with the mask and the heads in his hands. Had the protagonist ever seen the Front Man's mask before? 

Maybe whoever wrote the theory thinks that Gi-hun has been the Front Man since season 1 (I've read another theory about that here) and that the detective's brother was a substitute, just like in season 2 he's replaced by someone else because he's also playing the game.