Magic. There're spells in the world that can turn people into inanimate objects, seems like a spell that makes a fake wall appear is way within the realm of possibility. All illusory walls hide something, whether it's a bonfire, treasure or even a covenant. I don't think it's a stretch to think that someone from Oolacile, the land where the Chameleon sorcery was created, could've been exploring and found something interesting and was like "Fuck everyone else, only I know about this."
I love the image of scholars toiling away to create a spell that lets you turn into a pot. Dark Souls is a pretty weird game.