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 groups of the gigantic landmass. The use of "think ... although" indicates you disagree with how they identify their ethnicity. Thus I assumed you felt it was a problem.

Apologies if I read intentions that weren't there.