Here's the relevant small (fair use) clip from the article:
Except that the statement quoted is incorrect. You can but
most of the parts needed to make a gun, but the receiver is legally a "firearm" and you can't buy a completed receiver except by going through an FFL just as you would for a complete firearm.
You can buy unfinished receivers, but generally those require machining -- at least for handguns. I'm only familiar with 1911s -- I'm sure there must be unfinished Glock receivers, how much work is needed to complete those?
But the gun was a .45, and apparently only held six or seven rounds. My initial thought was "1911," but it seemed unlikely (not impossible) that a fifteen year old kid would have built a 1911 "ghost gun." And then there's this, from the MSN link:
The sources said the gun design appears to be unusual but emphasized that officials don’t know its origins at this time.