And thus for a very long time, and even today to some extent, the Japanese (or their cultural theorists at any rate!) have viewed themselves as the inheritors of "true" Ancient Chinese culture -- unsullied by Mongol/Manchu/Communist influence.

There's some truth to this. The Japanese preserved certain Tang Dynasty customs which were lost to China, and they have some of the best collections of Tang-era artifacts.

This line of thought was, however, used in very unfortunate WWII-era propaganda, like "we have to invade China to rescue its civilization, only we can do this!"