Yeah, I'm aware of that, however, most Chinese still consider themselves Han Chinese. So even if it is somewhat of a fiction, most people within China consider it true, which is what is relevant for the purposes of having a national identity.
My point is, they consider it true because of the country's political cohesion. If the country wasn't politically cohesive, the ethnic identity wouldn't be either. The unified identity is explained by the political homogeneity as much as it explains it.
I think the causality works both ways. If you have an ethnic identity, it's easier to assimilate new arrivals, especially when you are numerically superior. This has been the history of China for a long time.
