> Can you even make a firearm in plastic?

Yes but most designs still require metal bits. Typically the frame will be 3d printed and then you rely on machined components for things like slides and rails.

Article says:

  New York’s budget bill S.9005 buries similar requirements in Part C, sweeping   
  in CNC mills and anything capable of “subtractive manufacturing.”

So CNCs might be part of it. But I didn't check the actual bill text.