Stalin played both sides, but most of all, he played Mao and the CCP more than he played Chiang and the KMT.
Everyone, including the Americans, knew Stalin favored Mao over the Chiang for China. But since the US supported Chiang, and that Stalin was a major partner in WW II in Asia, the least that Stalin must do was to nominally support the KMT. Everyone also knew that Stalin were giving Mao and the CCP material support, least of all to fight the Japanese, but more to prepare for the possibility of a Chinese civil war for control of all of China. Stalin could not make those support above a certain threshold that would provoke an official American objection in front of all three partners.
Northeast China, or Manchuria, is larger than Germany, Poland, and France COMBINED, is rich in natural resources, and probably most important -- several excellent yr-round ports that even if Manchuria have little or no natural resources, those access to open sea would more than make up for that lack. Manchuria have always been vital to anyone wishing to have at least majority control, if not complete control, of China. The Japanese did not pick Manchuria for no reasons. And was it true that Nurhaci succeeded -- the Qing-Ming changeover -- only because he controlled Manchuria ?
So when Stalin effectively took over and literally looted Manchuria, by ALLOWING Mao and the CCP to use Kwantung Army leftover arms, Stalin could maintain the fiction that Russia wanted no interference in China's internal affairs while actually supported Mao with those Japanese arms.
I robbed Joe and gave you Joe's stuff, that mean I supported you. Simple as that.
No...But that does not mean you would not revise history, like you just tried.