Earlier quoted context omitted.
Someone married an Asian whose family happened to be ethnic Chinese, which made you think "huh these people are everywhere". (With regards to the diaspora comment) Then the comment, paraphrasing, "they think they're Chinese when they're not Chinese nationals at all". If their ancestors were from China, they don't "think" they're ethnic Chinese, they are ethnic Chinese. Unless you get into the nitty gritty clans and g…
I think OP meant he was surprised that the family felt a strong enough link to China to identify themselves as Chinese, even across time and distance.
Just as an example of something with a flimsier premise. Say a German American who doesn't speak the language, don't have known family there, have no loyalty to Germany, celebrate none of the German festivals, and the only indication left is a last name Schmidt. We don't say, "huh the German diaspora is everywhere" when these people say their ancestors are from Germany.