That's probably the biggest hole in the Koh theory, but it could be some kind of illusion projected by Koh through the mask.
Even if it isn't Koh, I think the spirit world, and the spirits in general, are heavily in play. Again, I can't believe Aang is Amon, so the only way that he'd be able to channel the ability to take people's bending away would be an infusion from something massively powerful.
His line at the Equalist rally about his gift being from the spirits, though most likely designed to stir up feelings of him being a 'chosen one', does ring true in my mind.