And now a howler with g3. Match over.
On chess.com GM Sam Shankland writes up his analysis of each of the games. This is what he had to say about that move:
"Now, the only explanation I have for the next move is that there are 2 versions of Nepo. He did not just blitz out g2-g3 without thinking and miss something obvious because he was not paying attention. He spent nine minutes, and still came up with a move that is very hard to understand from a player of his caliber. Sometimes he just seems to tilt really hard. I know from my own experience - I've played him twice. The first time I was on the black side of a Caro-Kann, and he played a fantastically energetic game. It was a wild fight with plenty of chances for both sides. He did not play perfectly, as nobody ever really does in super-complicated positions, but after a somewhat back and forth affair, he found a lot of best moves plenty of times, and eventually ground out a well-deserved win.
The other time I play him, it was toward the end of a tournament, I guess he may have been struggling with stamina, and he played some d6/g6/a6/e6 set up, moving pawns around in the first few moves without developing his pieces, and I absolutely demolished him with almost no effort. It was a weird feeling afterward, that I had a crushed a near-2800 so easily, and I can imagine this is probably how Magnus is feeling now. Every now and then, this seriously sub-optimal version of Nepo shows up, and unfortunately for him it came at the worst possible time.
23. g3?? Not only does this move lose violently, it absolutely forces Black to play the winning combination since everything else is tantamount to resigning."
Based on this, I think what DDB said about his temperament is apt. Sounds like endurance and mental stamina played a big part and as it dragged on and he fell behind he just didn't have it in him to recover.