It seems odd, but the way I read the demonhide revelation entry in the Inner Sea Magic guide, although this results in the flesh becoming as "tough as a demon's hide", it seems to grant an armour bonus, rather than a natural armour bonus. Have I understood correctly?
Yeah, it's an armor bonus. By the flavor of the revelation, it seems like it should really be natural armor, though. They probably made it regular armor to keep it consistent with other similar revelations.