If it's that, then it's pretty arrogant - I mean, that's like saying: "he's too stupid to make such a good move, so he must be cheating"? Even (relatively) bad players are bound to make good moves from time to time, either through dumb luck or through occasional sparks of inspiration, so that's hardly a reason to be 100% sure they are cheating...

There are moves and lines that only Super GM’s will calculate. Not IMs, not FMs, not GMs. It’s not arrogant, it’s the level of play.

When you are #2 in the world at something you are allowed to be a bit arrogant about it.

(down to #8 now, I suppose)

Not so much "outside his skill class" as "unlikely to be played by a human". Computers have a really good idea of what the optimal move is, and it's sometimes not a move any human being would play. The main way to detect cheating is to figure out that they're playing closer to the computer's line than anybody without computer assistance.

In this case, his off-the-board behavior, where he seems unable to explain some of the moves he made, is also suspicious. His skill set is such that there's no move he's not capable of playing, though he should at least be able to tell you what he was thinking when he played it.

Move in a critical moment, speed of decision for said move, cheating at this level is probably extremely hard to do it alone which already creates space for leaks...