But there's an interesting meta in that Magnus played far more passively than he normally would. And so I think he also expected he was probably playing an engine by proxy, and wanted to keep the position completely under control. If he knew the world was legit, they probably would have lost!
I'm still trying to reconcile how it came to be that the world didn't cheat though. Lowest common denominator amongst 140k+ people paired with inevitable chatter of 'Hey best engine move is blah' seems unavoidable.
But things like this are social. I didn't follow this (or even know it was going on somehow) but it seems very safe to assume that somebody and probably multiple somebodies were regularly pointing out and discussing engine moves.
So my only real assumption is that a significant chunk of people would end up deferring to the engine moves rather than their own preference. Of course my implied assumption there is also that a significant chunk of people were involved in the social aspects of this, but I think that's also a fairly reasonable assumption.
A very non-zero chunk of people also probably would not have even understood that that's cheating if it wasn't really clearly laid out in the interface somewhere. For instance computer assistance in the largest correspondence chess league is legal.
Any evidence of this whatsoever?
Here, for example, is one of the more well known and easy meta-indicators of cheating: Humans spend more time on difficult moves than on trivial ones. Cheaters will typically spend a comparable amount of time on a trivial forced move, and the start of an exceptionally deep combination. And an even bigger tell is that said exceptionally deep combo may be followed by a couple of forced moves, yet he will again take just about the same amount of time to play those moves.