But if he ought to take it seriously, because it was said with power and intention to make this belief widespread in China, would it then be worse than an insult against all chinese people?

It makes little sense. If it is such an insult to the Chinese people as you claim, how would the idea be able to persuade the vast majority of Chinese? I am pretty sure that I will not be able to persuade you of something that is both insulting to you and false.

Also why can't a black African be chinese just as much as any other chinese person? What if that black person was born in China?

The Chinese have a civilization going back thousands of years. It doesn't matter where you are born, just like I am not European for having been born here.