He's lost to weaker players before. He's lost to younger players before, and praised them. He's lost longer streaks. He's lost with white in such situations too.

He's even lost to Hans before, with white, in the game where Hans infamously said "The chess speaks for itself".

And he didn't immediately withdraw from Sinquefield. Hans' postgame comments, where he made an odd claim about preparation, came in between and may have been a factor.

Remember, Chess is a LOT about preparation these days, and Carlsen was also at one point #1 fantasy football player in the world. He probably knows a thing or two about applied statistics. Even overlooking the specifics, he wouldn't get as far as he has if he wasn't pretty good at keeping his frustration at losing from influencing his judgments.

This is the most enlightening comment I've read about this case.

He's lost to many players for many reasons, even during his reign as WC.

The thing about that loss, was that it was basically a curved ball, and Hans was prepared for it.

It sounds like Magnus was suspecting something and prepared obscure line to test his response. Its not conclusive evidence as there is always some chance he just studied that sideline and know how to exploit it.

But it looks like its conclusive evidence to Magnus or he knows more than he lets out.