I think Rogers's definition of a plastic vs. a metal mount differs from how most people think about those terms. For most of us, we're talking about the bayonet parts - the 'teeth' that lock into the mount on the camera.
The EF-S 18-55 on the left has a 'plastic mount', the EF 17-40L on the right has a 'metal mount'. Very few of us disassemble lenses, so we have no idea what's behind that mount surface. Roger is talking about how the screws that that attach that visible surface piece to the lens are connected - do those screws go into metal screw-holes that are attached to the frame of the lens, or are the screw-holes plastic?
'Plastic' can be quite strong, so for a 'light' lens (most lenses under 100mm, with the exception of the 'magic cannonball' 85L), I agree with Roger that I wouldn't expect any issues, and 'professional' could apply. However, for the bayonet 'teeth' of the mount, plastic wears down more easily than metal (vs. the screw-holes, which aren't subjected to routine 'wear'). That means a lens with a plastic mount (as I'd say is the common definition pictured above, not Rogers's use of the term), would be able to tolerate fewer mount/unmount cycles than a lens with metal bayonet teeth. Since a professional lens would be expected to last years and most 'pros' own several lenses and change them frequently, it makes sense to associate a metal mount (as pictured above, regardless of how it's screwed in) with 'professional' build.
I understand that, and even make the same (in my case unfounded) assumption about the plastic teeth wearing down more quickly, but would not Roger have noticed it so far if it did happen to be true?