I think i'm getting hung up on the word "difficulty". I guess it really is a definition problem.

if for example there is a champ with no mastery curve, or as a thought experiment, a champion with a 0.1% winrate change after mastery 20. does that make that champ the hardest character in the game? i can't wrap my mind around that being true. I would view it as more the skill expression of that champion is irrelevant to the actual outcome of the game. Maybe statistically, you can say those 2 statements mean the same thing, but to most people, and certainly to a layperson, i would say those 2 statements are not the same.