I am also confused by this statement about quality. Vitra, you could argue, is making the āoriginalsā as they own the license together with HermanMiller. Just in PP, not FG.
Personally, I prefer PP over Fiberglass, mostly because of durability and sustainability reasons.
Out of my experience, Vitra has always top notch quality. I am a huge fan of their blend of new and old design.
Granted, with a hefty price tag. But then again, in Switzerland, you wonāt see any ācheap Mexican laborersā though. Also worth something.