I don't really understand why he didn't just use trolls (which, as he mentions don't move in the day) rather than invent a new tree folk.

It seems like he came up with the disk idea and then thought 'this looks like a tree' and back-solved Middle-Earth and then was like 'oops, this behaviour is actually trolls' when writing it up (rather adjusting his paper to use stone discs)

I can't imagine there's many people who both get past the opening and don't understand the concept sufficiently for the allegory to be useful.