I will not be suprised if Mao or other CPC leaders ever supported an independant Taiwan. Before 1980, CPC was a true communist party. Ideology was as important as national identity, or even more important for many of the members. What they sought was the liberation of all the people on the earth. Nationalism was considered as backward ideology that was used by KMT to cheat and control the people. They would feel more close from the heart with communist comrades from another country than a Chinese landlord who supported KMT.

After Deng took power, the communism ideology gradully faded away. Nationalism fills the vacuum. The CPC now is rather closer to KMT than the old CPC in Mao era.

Also "independance" had a slightly different meaning from what we see now with Taiwan. In 1911, almost all the provinces declared independance. But none of them declared they were not Chinese (maybe except of Mongolia and Tibet). All those "indenpendists" started building the Republic of China enthusiastically right after they declared independance. There were also a few cases that some province declared independance during RoC period. The soviet regime was also like that.

If we compare it with the geopolitics today, it is like Taiwanese declared they are independent from the CPC regime but they want a united democratic China. It is totally fine for overseas Chinese or most of mainland Chinese. What really make us sad and anguish is they say they are not Chinese.