I don't think anyone's suggesting that an airline would misrepresent all their flights as being on A320s that don't exist and then claim to swap for mechanical issues at the gate to 737s that do. And they could do this on every single ticket. (Is that what people are suggesting?)

They're complaining that they could buy a ticket out of one airport on an A320, that plane could get delayed, and the airline could swap in a 737 that was waiting for another route. Passengers on the other route would then get the Airbus that they hadn't specifically filtered for. That might happen on one in 50 or one in 100 flights.