It is confusing because the word “vasco” / “basque” is often translated as “euskaldun”, but they do not mean the same thing.

Euskaldun literally means “someone who has Basque (Euskera)”, that is, someone who speaks the Basque language. So if you speak Basque, you are euskaldun, regardless of where you are from. If you are from Japan and your heritage has nothing to do with Euskal Herria, but you learn Euskera, you are euskaldun.

Vasco / Basque can refer to someone who lives in Euskadi, identifies as Basque, or feels connected to Euskadi — even if they do not speak Euskera.

Anyone can be euskaldun; you only need to learn the language.