You're making exactly the mistake I'm pointing out!

You do not print a lower receiver compatible with standard parts. The threat is a CNC milling machine cutting one from metal.

Ghost gun = key part made by sources other than a legal firearms manufacturer. Typically, this means CNC milling.

Printed guns are ghost guns, but the vast majority of ghost guns are not printed guns. Printing plastic has a very hard time confining the gunpowder. It's possible to print metal, but it's sintered (not nearly as strong) and the printers are still way beyond the home level.