As far as I know, the full and final meaning of American national is having American citizenship. We generally don't stop claiming our birthright en masse after leaving although the Chinese I happen to know don't stop saying they are from China either. Presumably Americans living in Japan would still be American citizens in contrast to a hypothetical country of "Americans" analogous to "ethnic Chinese" in any number of places around the world that they would need distinguishing from with the label national or citizen.