No, they're definitely Numidium's skin. This isn't a theroy, it's confirmed by MK.
EDIT: inb4 "mk isn't beth." He wrote a good chunk of Morrowind lore including ALL of its metaphysics and vivec related stuff. If he says this is how it is, then i'm going to listen since he was a dev for that game.
At least according to LadyNerevar, the bit about their becoming Numidium's skin is metaphorical. They definitely soulstacked their way to become the oversoul of the Brass Tower, just not the actual armor plating.