Well, culturally they are in large part. But the US has had massive immigration from countries other than Britain, which has caused a cultural drift (along with time and a big ocean). But think about what a place like India, S. Africa, or America was like before the British arrived, and look at them now. They are vastly different. Britain did indeed have a significant and lasting cultural impact on those areas. So if you're looking at it from the standpoint of culture, saying India is half-British isn't really ridiculous at all. The whole way Indians live their lives, see the world, govern themselves, dress, what they eat is all different now due to the British. If you're looking at it from the standpoint of race, then yeah it's obviously ridiculous. So if Paradox meant "culture" as a politically correct euphemish for "race" then okay I'm on board. Otherwise I would disagree with the notion that colonial areas were only marginally impacted culturally by the colonizing powers. The whole fact that they wake up to electric alarm clocks, put on business suits, go into cities in their cars, etc, etc... their whole lives have become "British" or "Western." 50% is probably too low!
But will there ever be any assimilation in Shanghai? Is China hardcoded to never assimilate?