The units in the Pusan perimeter weren’t National Guard, they were regular Army (badly trained and ill equipped, true), mostly initially from the garrison in Japan.
The NK army is way obsolete (1950’s-1960’s vintage for the most part) and their heavy weapons haven’t gotten much in the way of spares from the Soviet stocks since 1990. Their tanks and artillery would be massacred by the South Koreans if they tried to attack.
SK forces aren’t the semi-policemen they were in 1950.