GM Puzzle

Richard Rapport played this only winning move against Magnus Carlsen (but still lost).

It is ridiculous to lose from that end position. Unless it was bullet or Armageddon or something. Or, unless these moves were not played at all. After black's key move, white should simply castle and the position is still equal. But the poster likely forgot to tick the castling box

Well Magnus played Kf1 after black took on g3 but this was the idea of the move Bxg3

That would imply that ..Bxg3 was not a winning move as black's advantage is too small for that. And, .0-0-0 is better than .Kf1 anyway.. Unlikely that white would have lost his castling right at this point.

I assume the story is different though. Probably white's last move was the blunder .g2-g3? which threw away white's win and gave chances to black who saved half a point with his next move! Typical for a speed event or Carlsen must have had an off-day.

Magnus had already lost castling rights

Surprising but true! Less surprising is that the blunder occurred in a blitz game where anyone can make such mistakes - and still win

It's also a common problem with game diagrams. You can't see that the castling rights are gone so you assume they are there. Even when you check the FEN you can't rely on the poster to enter the correct castling state. They often forget about that when the castling move does not show up in the solution.