"...Now Magnus is required to play 1 move before resigning so he did so. He's basically saying: "I won't play with a known and admitted cheater, so I'm going to resign any game I'm forced to play with him" rather than doing a weird dance about skipping tournaments or forcing tournaments to not include Hans if they want Magnus to attend." - enfrozt
So yes, IF he was cheating, he likely would have started cheating on move 1.
I've definitely heard that knowing that puzzles are much easier for GMs/etc because they know there is a winning move in that situation. Checking the board periodically with an engine to know the evaluation would also let you know when there is a very good move to play.
Even at a very low intermediate level of chess, you can easily know an opening like this for 8 - 10 moves across 2 - 5 variations. I know cause I do.
Master Level Prep of the Kings Indian goes 20 moves deep across 5 - 8 possible variations. This has been demonstrated over the board several times.
It's trivial to find opening theory lessons on youtube on playing the kings indian defence covering 5+ counter lines going 10+ moves deep, even on weird lines. And then there are books about this opening.
There is zero value in starting to cheat on move 1.
Probably cheating after the 5 or 10 or 15 move if the opponent did something interesting may give someone an edge, but getting help in the first of second move it's unbelievable at this level.
Note that there is a whole page in Wikipedia about all the possible good possible second move of Niemann: https://en.wikipedia.org/wiki/Alekhine%27s_Defence