The printing does not really mean that much (I think)
One can very easily make either item in a basic machine shop (I could do it at school pretty easily).
It is just people not understanding technology.
Yes your are right someone could make anything in a shop. I could bend out some sheet metal and make a magazine on my bench. But a company with a few printers that made non firearm related products could turn out manufacturer numbers of magazines or lowers by a keystroke.