Well GP does say allows for such a state, not guarantees.
Well if that's what they meant, then heterogeneous melting pots also allow for advanced societies to work. And ethnocentricity is a shitty indicator for social cohesion.
> Plenty of homogeneous states which don't work.
Plenty? Name five, other than isolated microstate islands. I can think of very few modern homogeneous ethostates.