You can never be sure, but that is not important. The phrase used in court is “reasonable doubt”. There is certainly doubt, but is it reasonable? According to my database the position after 11. Nxd4 occured 29 times. The highest rated player to end up in this position was Frolyanov (Elo 2555) in 2012 against much lower rated player in a firstround game of an open. Needless to say, he lost.
Overall white won 19, lost 2 and drew 8 games.
It is quite funny that Mr. “Too weak, too slow” Fressinet managed to draw a rapid game with white against a guy with an expectation near zero. That is the expected reasult with 400 points difference. Could that have been collusion beyond reasonable doubt?
On a positive note: Nepo will not draw 8 games with Carlsen and go on to lose the tiebreak. There is no reasonable doubt for sure.