More than 1 in 15 Chinese people are not part of the Han ethnic group. When somebody makes big “mostly” generalisations about people, I don't expect “you could have half the clubs in a school composed of these people and still have some left over, assuming uniform distribution” to be true.
You do know that saying bad things about $CountryX doesn't prove good things about $CountryY, and vice versa, right?
Perhaps the ones you hear about are “ethnically mostly the same people”, but that's not actually true.