As symbols, these figures have come to riff pretty obviously on the foliate head iconography found on (mostly) English Norman churches. It’s generally thought that their original meaning is far from the ideas of the veneration of nature that are now applied to them, but I don’t think that’s important. It’s a modern, post-Christian interpretation of a common artistic expression, as is the modern understanding of The Green Man. On the cultural appropriation scale, I would rank it 1 (very low) to 0 (you must be joking). It doesn’t ask us to apply any archetypal mumbo-jumbo to some unsuspecting deity who will find it a poor fit.
Still, what a great new set of symbols. People like writing stories about them, and creating beautiful new art depicting them. So now you know. I’m completely and utterly out of the closet as a fan on the Oak and Holly Kings.