I don't understand this. Normally if passengers are told they are being denied boarding, their original booking will already have been changed.

Telling people that they are being denied boarding when in fact they were not is false and thus its effect is to deceive. Is it the idea that QR is not intentionally deceiving because how could they possibly have known that the original booking was still in place? No one could possibly have known that, it's not as if computers have been invented yet.