I don't know about 100 killing methods, but it is true American B-29 bombers obliterated every city in North Korea until there was almost nothing of economic or military value left standing. Carpet bombing methods were used, as it was customary in those days. Much of the urban population of North Korea was wiped out.
Monster Zero, first, there wasn't much of an urban population in North Korea in 1950. Now, the North was, relatively speaking, the more industrial half, thanks to the Japanese, but Korea as a whole was largely rural. Second, between 1945 and 1948 there was a shift of population between North and South, as well as between Korean residents of Japan to the North and South, as ideological lines between the emerging Koreas jelled. Much of this manpower lived on the margins of the cities, and went into the two Korean armies and their auxiliaries prior to hostilities. Third, except for some port cities on the east and south coasts and Taegu, the great majority of Korea's population was concentrated in a series of narrow plains on or near the West Coast that ran from Pyongyang down through Seoul to Pyongtaek and south. This was the axis on which most of the fighting took place during the North Korean invasion. Again, the cities were rather small affairs and not what the USAF had seen in the European or Pacific wars. But, since they were important to Koreans, needless to say a very great amount of damage was done by the two Korean Armies before the U.N. forces ever got into the war.
I believe the 'carpet bombing' you refer too relates to U.N.C. attempts to keep the rail lines cut and impede North Korean logistics after Chinese intervention. Now, by war's end, the North Koreans had rebuilt their armed forces, and developed a logistical support for armies in the field based upon massed manpower carrying 80 pound loads. (The South used one similar, titled the Korean Service Corps). By the Armistice, the North Korean Army and logistics were at a point where they could conduct a Corps level attack, be repulsed, and still move the rations and war material to sustain a corps level counterattack.
If your assertion was accurate, the North Koreans would not have been able to sustain the additional two years of war effort that they did.