It is my understanding that both sides had conflicting goals in Korea.
NK wanted to unify with SK by force. So NK lost, by that definition.
China wanted to salvage NK, but did not see over-running SK as much of a goal. China achieved success in keeping NK from total defeat, so China could be said to have won.
SK wanted to remain independent, so they could be said to have won.
The U.S. was conflicted. President Truman was content to keep SK independent but had no desire to over-run NK. General MacArthur wanted to over-run NK and keep running through into China to overthrow the Communists, who had only been in power for less than a year and still didn't have full control of Tibet or Xinjiang. MacArthur had no love for Chiang Kai-sheck, I don't know what he would have done if he did defeat the communists. He also wanted to use atomic bombs against China.
Truman has been looked upon favorably by historians, but was pretty unpopular in 1950. MacArthur has been looked upon with mixed feeling by historians, but was very popular in 1950.
So MacArthur used his popularity to pressure Truman to allow UN forces to move right up to the Chinese border, despite warnings from China and Russia not to do so. Truman worried that China or Russia would intervene, but MacArthur felt otherwise and pressured Truman into allowing a strategy (total defeat of NK) that Truman didn't like.
When China and Russia intervened, MacArthur was discredited and forced back into retirement. The U.S. reverted to Truman's preferred strategy, which was to hold SK but not all of Korea.
MacArthur's strategy was defeated. Truman's was not.
Call it what you will, it is more of a semantic argument than anything meaningful.