I’ve done a fair bit of walking in the hills of the Northern UK with very experienced people (myself definitely not included). I’m pretty sure that if there was a form of killer vegetation out there, someone would have mentioned it to me.
I have come across areas on hilltops with wind polished ice that was very nasty stuff to try to remain upright upon, and which could have caused you to slip over cliffs and so on. I think the “verglas” explanation sounds good.