[chongli]

have them play blitz (where the moves are way too fast for info transmission to happen), and see if the skill level scales accordingly?

Not a fair contest. There are plenty of top classical chess players who are weaker in blitz and vice versa. It’s a different skillset. Classical is all about preparation for the opening followed by some deep thinking in the midgame. Blitz is all about pattern recognition and the ability to simplify down to an ending where you can blitz out the exact solution from a database.

[vintermann]

It's true that some people are worse at blitz, but if Niemann's OTB blitz rating is as good as his OTB rating at slower time controls, that's evidence against him cheating ... at least in ways that materially affect his rating. I guess it would be possible to cheat with an engine tuned to your actual rating just to make it less stressful.

[Taek]

Well, even endgames with as few as 5 pieces on the board are beyond what a human can solve with memorization. I don't disagree that blitz is mostly pattern recognition and rapid tactical thinking, but that applies all the way down to the end.