That is the point, though.
There's no and can't be no skilled play in D&D, because the only possible skill being expressed is players' ability to please the GM who pretends to be impartial. Some even go further and gaslight themselves into thinking that they are actually impartial, and their decision-making is predicated on anything other than their left foot.
I don't think anyone really thinks they are impartial. The idea is to strive for being as evenhanded and fair as you can be. It is a goal, not an end state. And there is a substantive difference between a GM who is trying to do this (especially ones who are humble enough to admit when they've made an error or a questionable call) and ones who just see themselves as the guy in charge. Now you are obviously not measuring player skill in a vacuum, and it isn't the same as testing it in a more objective environment but a GM can very easily make an adventure or a sandbox designed to be challenging and designed to reward players who are cautious, strategic, creative, diplomatic, tactical, etc. Yes the GM and the system are the two things that will be measuring the players but like I said before plenty of sports with scoring measure skill subjectively (even something as concrete as boxing requires some subjective judgement when awarding points to determine who won each round----you wouldn't have split decisions otherwise). Is it a perfect and flawless system for measuring player skill? Of course not. But I don't think anyone is arguing that it is.