I think that statement is based on a fixed selected ISO so if you choose a shutter speed and then the camera needs an aperture that is either larger or smaller than the lens can go you get an incorrect exposure. Same thing can happen with Exposure Compensation, you can bump into an aperture need that can't be achieved. If you are using auto ISO then it should not be an issue...most of the time...
PHRubin wrote:
I don't understand "when you can't move the aperture". If the camera can't set the aperture, something is wrong. Shutter speed preferred means you set the shutter speed, the camera picks the aperture needed for correct exposure for the ISO in use. If all is too dark, the ISO wasn't high enough for the speed chosen and available light.