You're shifting the goalposts from "they're ethnically the same people with a shared culture" to "by the standards of large landmasses China was more cohesive and united than India".

Comparing Chinese unity favorably to India is damning with faint praise, and doesn't do anything at all to help your original argument that China has better "social cohesion and political stability" than Western countries by virtue of having less immigration.

We're not comparing the social cohesion of landmasses, we're comparing the social cohesion of states, and India is a particularly disjointed example to use.