Real life conquests never result in referendums. Victors occupy the land and make it theirs. In China's case, most of the land absorbed into Chinese territory occurred during Yuan and Qing rule. The Hans after the fall of Qing just took over the spoils.
Should California or New Mexico have a referendum whether they should go back to Mexico? The immigrant Yamato Japs pushed the Ainu northwards into Hokkaido - should there be a referendum today whether Hokkaido should be independent? When the Monggols conquered all of China and Eastern Turkestan, were the Hans, or for that matter the Uygurs given a referendum? Aborigines in Australia? We can go on and on, but you get my drift.
If you want to use ethnicity to determine land ownership, then the whites should all fuck off from the Americas, Africa, Australia and NZ and return the land to the natives.
As for Tibet, Qing China ruled Tibet from 1790 to 1912. One could claim the CCP was recovering lost territory.