Wait why is that a problem? Nationality and ethnicity are two different things.
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.
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.
Consider the way Russians-in-exile perceive themselves in the current geopolitical context.
This is super common in Europe. We have people who think of themselves as X and speak Xian as their mother tongue even though the region has been part of Y country for centuries.
Slovenians, for example, managed to survive under foreign rule for about 1100 years before re-gaining independence.
Nationality and ethnicity are very different concepts.
for a more American example: Ask any of the indigenous peoples how they feel about their American, Canadian, Mexican, etc. nationality.
> Consider the way Russians-in-exile perceive themselves in the current geopolitical context.
Those who I've spoken to identify themselves as Russian citizens temporarily in exile, whilst I am ethnically Russian who has no connection to the country at all. I don't think these are the same.
You have Bretons that are ethnically close to Irish/Scotish, Alsaciens who are ethnically Germans, Basque who are similar to the Spanish Basque, Corse who want to be independent...
A better example of a country that more or less encompass an ethnicity would be Germany, or Japan. But even there you'll find exceptions (Ainu in Hokkaido, Okinawa being also ethnically different as Ryukyu vs Yamato...)
The fact that it has substructure and blurred lines doesn't mean it isn't a valid abstraction.