He's not talking about the Medusa's Head. He's talking about the GoW AP chest that's a really rare drop. It's pretty much just like the R&C AP chest but filled with a ton more AP. But I never thought about how it's only been seen on Metropolis. If this theory is right, then it shouldn't really be called an item, just part of the stage. Because it acts just like the crates and exploding cubes do in that level.