They didn't really rule that the string was a machine gun. They sent a badly proof-read letter saying words to that effect, but their intent was to declare that a rifle jury-rigged to fire automatically (using a shoe string) was in fact an automatic weapon.

And I doubt they will declare the manufacturer of the lower receiver is manufacturing a weapon, if it's done in such an automated way. They will just change their rulings so it's a crime to order such a part. If they can't do that, they will work with whatever federal agency regulates telecommunications, and it will suddenly become a serious crime to order certain gun parts from a 3D printer.

Laws don't always change faster than technology, but they change pretty quickly when there's the right kind of pressure.