I, too, was pretty sure I'd seen printed receivers before, and of course plastic ones have been commercially available for years.
I suspect, however, that HB's is considered a "first" because it's essentially ready-to-use right out of the printer. Other 3D printed ones were most likely out of weaker resins (a lot of stereolithography-type stuff is meant only to check fit and appearance, as is far too weak to actually use) or too "coarse"- not close enough tolerance- or needed significant finishing (surfaces milled to fit, holes drilled, etc.)
Just guessing, but it looks like HB's might have only needed the grip attachment screw tapped in order to use. (He said the buffer tube threads were modelled, but he didn't mention the grip screw.)
So presumably "first" as in the first one that's basically plug-and-play. Print, assemble, shoot.