This implies that if you make the lower receiver (the part that gets the serial number, which is the "firearm"), and then buy the trigger group and upper receiver and install them, then in theory it should be perfectly legal.
Actually, I believe you can even sell them, you just can't be in the business of manufacturing them for sale. So ATF's not going to believe you if you say you made 100 last year for your own use, then changed your mind and sold them.