So? That doesn't mean that the Chinese can win. For a comparison, the U.S. threw tons of men and materiel into propping up South Vietnam, but it still lost that one, obviously. The Soviets did the same in Afghanistan and only managed a stalemate...there are lots of examples of this on both sides.
To get concrete, have MacArthur die in a car accident a few months before the war starts. His replacement is less aggressive and moves more cautiously than MacArthur did IOTL, both delaying Chinese intervention (the U.N. forces aren't racing for the Yalu and MacArthur isn't talking about nuking Manchuria) and ensuring that when they do get involved they run into well-prepared defensive lines instead of strung-out advances and get massacred. U.N. weight of arms allows a gradual push northwards over a few years, and ultimately both China and the Soviet Union throw in the towel in supporting North Korea because it controls too little territory to be a viable state, they're both taking large losses without anything like corresponding gains, and they see bigger opportunities to bloody the United States elsewhere. Ergo, Korea is unified (or at least might as well be, with the Communists controlling nothing but a narrow strip of land along the northern border)...but the U.S. only won one round, it's not like it won the whole Cold War or that the Chinese and Soviets don't have other ways to get back at it.
