Certainly but what I meant is that for the past few hundred years, at least, the Han have seen themselves as a generally related and unified group, despite dialect and regional differences. In India the different Indian ethnic groups generally did see themselves as different ethnicities altogether.
Then the question becomes whether the ethnic homogenisation led to the state centralisation, or the other way around. The ethnic homogenisation of various ethnic groups into the Han was definately a long and slow process and the modern Han ethnicity definately contains many many traces of other ethnic groups who have disappeared or significantly reduced.
To follow Jared Diamond, it is the geography of China that allowed for both the homogenisation and the centralised united state (and those factors both influenced and helped each other). I am not a strict geographical deterministic, so I wonder if a similar process is not impossible in India.
Any connection with the caste system? By placing different ethnic groups into different castes, these ethnicities endured in India while in China they were absorbed into the Han melting pot. Perhaps a very early PoD wherein the caste system develops differently.
As it is said that the caste system originates with the Aryan invaders, perhaps an enduring Indus valley culture? Thus the Indus valley becomes an analogue with the Yellow river in Chinese history, while the Ganges plays the role of the Yangtze. These river cultures unite into a single unitary culture, and slowly assimilate the peoples to the south over the centuries into a more-or-less ethnically and culturally united bloc.
Would it be too cheeky then to reverse the roles of China and India, sending Aryan hordes to destroy the cultures of the Yellow river and leading to the development of a caste-based civilization that refuses to unite for very long along the banks of the Yangtze? We couldn't labour the analogy too long, as a China-esque politically unitary and culturally enduring culture in India would have huge effects on the rest of the world. It might be interesting though.