A reasonable, if somewhat short, overview of the topic, although a bit generous when discussing the air power situation over Korea. Smashing older Russian SAM sites is almost an American pastime at this point, and I have serious doubts the KPAF would put up anything beyond token resistance in the air. The author also skims over the massive leaps in lethality on the modern battlefield. It's difficult to stress this enough, but T-54's and T-62's do not have a long life span under these conditions.
So the DPRK would need to achieve total surprise, fight ~20 miles to Seoul against an opponent who is equipped with mostly modern weapons, and who has planned for this very attack. They also need to do this with no air support, little to no armored support, and limited night fighting capabilities.