Calling it a strong enough link is giving it too much credit. It's more of an acknowledgement of lineage. Not difficult when names, festivals, languages are retained. In fact, if you're within 3 generations of emigration, you would even know the province and town your great grandparents came from.

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.