They didnt enter the war when pyeongyang fell, they entered when macarthur kept pushing to the yalu river. They actually reached the yalu river in fact...which is the border with china. So China had this huge UN army filled with western imperialist pig dogs pushing right up to their border.
They knew it would provoke the chinese. Churchill wanted them to stop at pyeongyang..but good ol macarthur
It was a hugely different situation.
MacArthur wanted to use nukes as well and was replace when he kept pushing the matter