Ah Yes. The Comjunists under Mao did stand back and let the Nationlists carry the full weight of the war. I do not think that it would have changed the outcome much though except that the Communists would have risked a full out fight with the Japanese that would have devastated their numbers and possibly prevented victory in the Civil War. Again unlikely. It may have postponed defeat because of USA failure to support Chiang and Soviet support of Mao.
The point though is well taken and was also well understood at the time. The Communists hid their strength and ability until it suited them and the Japanese were then happy enough to leave them alone. Even then it still took a four year civil war to knock the Nationalists out of China.
Unfortunately, someone had to win that civil war. Its end marked the end of two generations of internal warfare in China. It was then followed by two generations of Comunists command economics. Now it has been over for essentially 35 years. It will take the end of the Communist party for a proper history to be taught.
How the Chinese Regime Gets Away With a False History of World War II
While the Communist Party was indeed militarily insignificant at the start of the war, it built up a potent and well-organized army in short order, as reflected in the single major military operation carried out by the communist forces—the Hundred Regiments’ Offensive of 1940. Mao, however, criticized Peng Dehuai, who led the successful campaign, for revealing the strength of the communist troops. During the Cultural Revolution (1966–1976), Mao would use Peng’s “betrayal” as part of the excuse to have him purged.
Predictably, among the most primary of these are the Japanese, as evidenced in anti-Japanese riots of recent years, where even Japanese-brand cars have been overturned and set ablaze. In this narrative, China is a perpetual victim. Apologies from Japanese leaders are considered insincere, and the small but vocal far-right wing of that nation is gleefully portrayed as actual Japanese policy.