Its trade names over material science.
Glocks were made from Polymer 2, believed to be a nylon reinforced with glass fibre.
This has caught on and manufacturers don't like to say plastic, when there's a bit more to them.
Airsoft guns don't need the strength of posh plastics to replicate the weapon and some of the more basic plastics do fine.
People like to say polymer instead of plastic, cos it sounds better I think, but not all polymers are plastics, all plastics are polymers.
It's hard to mislead anyone if they don't know what the 2 things are or the difference.