Not true. Sunset is not only shot when the sun is still in the sky, but just after it goes below the horizon, where light will drop significantly. This time is the best time to get a nice mix of blues and oranges in the same scene, like the sample I will include below. However, shutter speeds can drop very low, especially if you are keeping your ISO to 100 or 200, so especially if you are doing HDR, a tripod will be most essential.