Re: All very true, but..
"It's a first go, and it's a proof of concept."
But it's not really, you could make the same product by injection moulding and we don't have a plethora of plastic guns being sneaked onto planes etc. Guns are necessarily made out of metal to have any sort of impact.
"You could easily design most of the parts for a 3D printer, and use a metal pipe for the barrel."
You could, but as the article points out you could do all that without the 3D printer.