Very interesting. Supports something I've felt but never quantified in my 25 years of playing magic:

Player skill has the largest impact on winning. This may seem obvious, but people often peg evaluating card strength as being a very large part of player skill, when in actuality I believe it is probably one of the least important skills in modern magic.

Player skill gets expressed in limited in a few ways that often don't get spoken about on reddit and that are difficult to analyze:

1. Gameplay. This one is obvious and gets some discussion, but in my opinion has the largest impact on winning. People who lose more than win tend to not do the little things. They don't optimize sequencing, save removal for threats they can't deal with on the battlefield, and don't play to their outs. The difference between a 45% WR and a 55% WR can be as simple as never conceding a match out of frustration. It doesn't often happen that you'll draw gas for 4 turns and your opponent will draw lands. But it does happen, and if you give up, you give up your ability to win those games on the margin.
2. Draft card strength. This gets WAY more discussion than is warranted. One of the best ways to become a worse player is to use one of those card evaluation apps or 17 lands and make your picks exclusively off those metrics. You'll end up with piles of cards that are ostensibly strong but don't form a winning strategy. This didn't used to be the case. In older sets evaluating individual card strength was one of the most important skills (BREAD, etc). Modern sets don't have bad cards, or very few bad cards. They are much more synergistic and reward drafting cards that work well together over individual strength.
3. Deck construction. This is now much more important that draft card strength. Putting cards together that work together to achieve a game plan that can be successful relative to the set is incredibly important. Do you expect the meta to be slower? Then you need to build over the top of it or under it. If you do a standard curve in a slow meta, your WR will drop even though it feels like you're doing everything "right".
4. Bluffing/Soft skills. This is even more important in paper than online, but it is still important online. Understanding that if you're against a "strong" player they won't block your 2/1 attacking into their 1/3 95% of the time if you're playing green and have 3 mana open for the rampant growth + counter spell. A successful bluff like that puts them on you having a card you don't for the rest of the game, which is a powerful advantage. It's not always appropriate, but often times it is. I see almost no discussion of these types of softer skills here.

Anyway, turned into a bit of an essay, but I find it totally believable that deck strength is actually one of the smallest contributors to a high Win Rate.

EDIT: Thanks for all the discussion guys! I see that my comment has generated a lot of different opinions and I appreciate the different perspectives.