I have a hunch that this gives the poorly trained and poorly fed NK troops with poorly maintained equipment a bit too much credit. I have a hunch this underestimates the people of Seoul.
Your hunch would be wrong. North Korea has tens of thousands of short range artillery pieces, rockets and self-propelled missiles on the border 35 miles from Seoul. They are low-tech and would very reliably reduce Seoul and millions of people to rubble within minutes of any attack on North Korea with absolutely no way to stop them. This would occur even if North Korea was somehow unable to use any of their dozens of nuclear weapons and if all of their subs, planes and ballistic missiles failed.