The sense I got from the book is more that we could not influence events on mainland China in any important way period. The Japanese invasion was a complication for both the Communists and the Nationalists, but both camps figured that in the end the Japanese would be defeated by the allies so they wanted to save their strength for the inevitable post-WWII civil war. Both camps wanted U.S. economic aid but without strings. I don't remember that Tuchman made a case that we would have done any better pouring resources down the Communist rathole than we did pouring them down the CKS rathole.
I agree that Tuchman thought we would have no lasting impact on China. But I thought she was of the opinion that Mao was willing to actually fight the Japanese, while Chiang was hoarding the resources we gave him. Also, I think she thought that our support of Mao would have brought us a rapproachment with Communist China, instead of their hatred. That premise I doubted. They had nothing but contempt for us, partly because they were communists, partly because they were Chinese.