I agree that disc height is relevant, I was taught that way and have used it until now.
I also agree that Pilots actually judge height (above the ground) by 'eye height'.
I further agree that if we are to teach eveything else (hovering, level skids before engine off etc.) by skid height then it would be sensible to have a common 'currency'
for the student and use skid height as a guide for the development of GE.
I agree with everyone...?!!
As has been said, this happens every time someone tries to make a simple rule.
Maybe I'll go with the alternative I outlined earlier....
"GE begins from a skid/eye/disc height
of somewhere up there...!"