Typically a gun assembled from parts would still have the lower receiver stamped with an ID.
The shooter in the article probably built a 1911 from an 80% kit and the kits are getting easier to finish. The virginia tech shooter used two pistols I believe.
Just stating that some pistol kits are very easy to finish as opposed to an ar/ak
The ATF raided some shop in CA specializing in polymer lowers. Claimed they were too easy to finish, though they still met the 80% standard.
This stuff isn’t rocket science.