This isn't anything I lose sleep over. 3-D printers are advancing rapidly in making plastic components. Maybe in a decade, someone will use 2-part high-temp epoxies as ink and you can make, basically, a J-B Weld pistol.
But people have been able to make their own guns for centuries. Not highly-accurately, high-powered guns, but firearms - yes. I could cast iron, brass or bronze in a sand mold from a wood pattern and make a bunderbuss, mortar or canon. A length of pipe can be made into a "zip gun", and with a lathe, well, you can make anything with a lathe.
So really, the new aspect is that you might make a minimally effective firearm with parts from OfficeMax rather interacting with any other, actual, gun-owning rednecks at the machine shop, welding supply or plumbing store. So I can imagine a white-collar worker (i.e. journalist) imagining for the first time that he could make his gun and fear that his neighbor might, too.
I'm not a machinist. Or a welder. They can do many things I can't. But being proficient at plumbing, wiring and electronics mean there are always snowball canons, rocket launchers, fuel-air explosives, and solar death rays around the house.
"Don't try this at home." isn't an instruction, it's a challenge.