If you look at comms videos on yt from teams, you can clearly hear the crowd
"BuT tHeY cAnNoT hEaR iT aS tHeY aRe WeArInG hEaDpHoNeS"
Bro the sound is being reflected through a player's mic which the whole team + coach listens to, they can all know the crowd is making noise or not because Mics are not noise sensitive. Even with Noise reduction, such high levels of sound are not removed completely.
With all 5 player mics throwing in the crowd sound and the fact that the noise cancellation doesn't cancel all the sound required (personal experience, i've used the Logitech G331 and my dog's barking from the mfing garage is audible inside those and I live on the 2nd damn floor fgs), crowd cheating is a real thing.