The reason they think it is an unused boss is because you can find it through an enemy modifier code. Those backrounds have no specific enemy assigned to it, so the game randomly pulls out random text, which is the reason why it says “The Sign attacks!” and other random stuff. When you’re targeting the random enemy, you’ll see a black square, proving further that you are targeting an enemy, not the backround.

Try assigning an enemy to that backround (like the Masked Man) then you’ll see it looks more like a backround used for an enemy. The question is, WHAT enemy.

I saw a Youtube video explaining a theory on the unused backrounds, but I can’t find it at the moment.

Lastly, there is a code to get to a debug room in this game, and I can confirm there is one, since I’ve been there.

Will post the theory video and an image of the debug room once I get the chance to.