IMHO a huge aspect of Carlsen mental strength isn't just the focused, at-the-game part, but we just see him enjoying Chess in many angles: not only he plays all styles, he streams relaxed, he plays Lichess and Chess.com; Chess is not only his job and passion, but it seems that he's also able to relax while engaging with it.

The only top-athlete that I see do the same is Max Verstappen, who is know to play competitive racing-sims online even hours before a real F1 race.