Not a euphemism. They're made out of polymer. This is pretty direct.
you don't have to accept if you find polymer cups ridiculous
I don't and I don't know why anyone would be upset over that.
You said they aren't called polymer notes, I showed you they are called polymer notes by linking evidence. What else is there? Are you still saying they aren't called that or does it just upset you for some reason? You can hate it but it's still true.
Polymer cups are also made from synthetic polymers like polypropylene. So are polymer knives and polymer forks.