I don't think it's merely mental albeit it seems like even nervous Carlsen is cooler than his very focused opponents (see game 3 vs Fabiano where caruana had a completely winning position after carlsens blunder).

Carlsen has spent the core of his career mastering two aspects historically underlooked aspects of the game.

The first is the endgame, and there isn't much to say there. He's by far the best end game player by far and it's not even close.

The second are drawish locked positions where most GMs can't but see a draw. Carlsen realized that in order for it to be a draw his opponents still have to play perfect and he focused a lot on accumulating small but convincing advantages in those kind of games.

Another thing that should not be overlook: mental strength, like you point out.