Bah. We've seen how devastatingly effective our "reliance" on precision airstrikes can be. That, coupled with what would have to be extremely dense masses of North Korean troops, will result in the greatest military slaughter in human history.
The ground invasion of N. Korea may not be easy. However, repelling N. Korean invasion is not as hard as he said. Because the invasion force will be trapped in narrow invasion corridors. You are right.
Infantry, well, they could either die on the roads, or spend a few weeks walking over those hills. Except that they don't have weeks worth of food, water or ammo. Once they leave their bunkers, that will be the end of their resupply. 800,000 infantryman need a lot of food, and they won't be scavenging jack out of what they just shredded with artillery.
Their air force and navy will be swiss cheese by day 3. Their army will be able to inflict a huge opening shock, but they'll run out of steam very quickly.