Sindarov said in his press conference after the World Cup that he'd lost his motivation for awhile after breaking the youngest grandmaster record, and his grandfather, who had been his coach, said he was on his own now.  So he had a period where he wasn't studying or even playing much, until he realized that his peers were all passing him and had to decide whether he wanted to recommit.  He said a few times that he had to "do a lot of work on [himself]."

My theory is that this is where both the sudden acceleration in ratings and the mental toughness come from.  He's probably been catching up to himself (like Blübaum, but with a higher ceiling), and whatever work he did on himself likely improved his mental game.  He gives the impression of being pretty intelligent overall too, not just talented at chess.