In theory, you're correct. The idea that skill with champions can counteract poor balance decisions is a true idea.
In practice, this game is balanced so poorly and the skill floor on most champions is so low that it doesn't matter; you can pick up a new champion in a few games to the point that any residual difference in skill is far overcome by the difference in balance, for almost all champions in the game. There are only a few champions that exhibit a win-rate difference with practice that is at all comparable to the kind of WR differences that arise through poor balance choices.