> It is my (possibly outdated) understanding that even today they make distinction between descendants of Japanese born abroad and the "true" Japanese, to the point they have different words to tell them apart.
And the USA distinguishes "natural-born american citizens" from "american citizens". The law in America forbids a foreign-born citizen from being president.
Similarly, English also has a special term, a "natural-born citizen", to distinguish this difference. Having different phrases for two different things doesn't really mean much.
By contrast, according to the wikipedia page on the subject (https://en.wikipedia.org/wiki/Foreign-born_Japanese), Japan has no restrictions on foreign-born citizens. They can become Prime Minister and hold other political offices. The difficulties lie in gaining such a citizenship, not in what you can legally do once you have it.
I do think there are valid points you could make about how Japan's policies and culture have resulted in relatively low foreign immigration, but the talk of "having different words for things" and types of citizenship doesn't further your point nor seem based in fact.