In an RGB scene, why can we assume color difference images are band-limited?
It isn't the scene that is bandlimited in the color channels, it is the human visual system. The color-sensitive cones are not nearly as dense as the intensity-sensitive rods in the retina.

Therefore, the argument is that it is not necessary to preserve as much color bandwidth when transmitting or storing images or video, because the human visual system can't take advantage of it anyway.