Hence the technicality. If you are in first person and shoot any thing be it a gun or a magic stick spewing fire, you are playing an FPS. The only reason why games like Skyrim aren't FPS's, and the only reason you should cite in any legitimate debate, is because the developer says it's not. That's the long and short of any genre identity for a game. The reason I mention the mod is because most gun mods to Skyrim are adding little more than a 1 MB retexture of existing assets. Everything else is already there, which demonstrates the distinction is contingent on something miniscule and possibly arbitrary.