Who else noticed the flag of China? It should be called ROC or Taiwan if you use that flag.
During WWII, that was already the flag of the Republic of China, which happened to be the only "China" in existence at the time.
No other flag was used then, so it would be anachronistic to use communist China's flag here, and Taiwan was not part of China at the time so it would be even worse to talk about Taiwan fighting the war.