The constitution is, unfortunately, neither vague nor on our side when it comes to imports. You could make the case that once it's in that state, it's out of the Feds hands, but if MGs would muddy the waters, a once-imported then remanufactured firearms are going to turn those waters positively black.
since demilled kits are not firearms, there is that pesky issue of what firearm is. Remember the "shoe string is a gun" ATF classification. Well, there are plenty more cases in the "grey" when we are talking about literal bits of metal, some of them were never firearms, but ATF says that they are. The whole "raw material" makes no sense. Coal was some micro-organisms that may have traveled from other "states". How long does an item have to be in-state to be considered a "raw material"? To some extent content of scrap heaps and junk yards are "raw materials" for something. When agencies like ATF and lawyers come to life, simple things in life suddenly become complicated.