I watched Sindarov's postgame interview
https://www.youtube.com/watch?v=qr8LV1aS5b8
and he had prepared the Marshall Gambit line with 12...O-O for last Fall's FIDE World Cup.
Even though 12...O-O looks reasonable, it had never been played in a high-level game. Nakamura's postgame interviews stated that his preparation did not include that move.
[In the muckraking department, Hikaru's livestream directly after the game could have been interpreted as throwing his team of helpers under the bus, but his later youtube recap was more diplomatic.]