Why? Rough Terrain means that Allied Aircraft doing CAS can use ground cover/clutter to mask their approach and reduce the amount of time AAA has to detect, target and fire on them.
I think he's saying that thinking that NK anti-aircraft fire will be harder to dodge in a confined environment. He's right, of course, but you are also right in saying that the NK forces will have a lot less time to get a track on them. SAMs would be tough to use if the attacks are planned well. And GPS-guided bombs (like the Paveway series) don't require the airplanes to be very low, either, they can drop from high, out of the range of NK AAA. You better believe that US forces will have EW support coming out the eyeballs and will mop the floor with the KPAF, so the air will be their, whether the North Koreans like it or not.