You have to consider match vs tournament play. The two are really very different. In tournaments you're playing a small number of games against a large number of different opponents, of often dramatically different skill levels. The large number of players also often also all but guarantees that some of the players will be off form, and easy pickings for the rest of the crowd. It's a really fun and interesting dynamic but very different than match play.

For the world championship players spend the better part of a year, generally with a large team of other world class players, intensely preparing to play games against only a single player, studying their games, and finally unleashing absolutely everything they've discovered. And then there's a constant metagame knowing each game you play one day will be deeply studied by your opponent and his team, and he will come back fully prepared the next day.

Carlsen gave up his title because he no longer wanted to put the massive amounts of work in that are required to maintain it. Would he be the best match player if he started putting this work in again? Maybe? But he's chosen not to do that.

I genuinely think Ding could beat Magnus is the WCC 14 game format. Magnus does better on tournaments but 14 games against the same player is a different beast and while Magnus has performed well in the past he has also shown plenty of weaknesses in the format.