He did it again in game 11. Some commentators on chess.com were saying that it was "a typical Magnus joke" or something, that they expected it. One could argue that he did it on purpose to make the "adjustment" in game 9 more believable.
Technically Nepo could have appealed but who knows what would have happened. Nepo could probably come off as the nagging, sore loser player.
Carlsen was asked about this at the press conference and he said that "that was normal", that they are just adjusting the pieces and have no intention of moving. Admittedly, it doesn't seem to be a big problem for top players as long as the player touching the piece shows no intention to move. I have seen on Youtube some GMs adjusting pieces without saying anything, and their opponents don't complain, but of course it must be something exceptional since they are not doing it all the time, as some park player or beginner would.
I also suppose that in this particular case, it would be a bit awkward for a player to say "adjust" out loud when their opponent is not sitting opposite or is not around, but they should remember that there are cameras everywhere, plus this is the WCC! It's a serious thing.
Very entertaining way to report and tell this story, @lighteye.