It looks like the 3D printer mentioned is in fact a 3D printer the photos in the link below, show a box for a Dremel Idea builder, a variant of the Flashforge dreamer, which can only print in PLA. So even though they were most likely printing a lower reciever, it probably wouldn't have lasted very long in actual use.

The search warrant was served at Chai’s house on the morning of April 28, 2015, and the deputies say they discovered the “3D printer” creating a lower receiver for an AR-15, which is an integral part of the assault rifle.