44 Amp explained it well.
Once a firearm is OFFICIALLY de-milled, it's no longer a firearm. Hence the guys that take de-milled guns, and build them back into functional weapons, are making their own firearms. Perfectly legal.
I've also seen bolt actions- like a PH M85, non-serialized and incomplete. It's not a firearm. One could heat-treat it, complete it, and build their own firearm using it.
I do find it interesting that anyone that can legally own a gun, can build one for himself. Fascinating hobby for those with the machinery and the knowledge.