Well, the English is certainly native level and they appear to make none of the common mistakes second language learners do, like forgetting or misusing 'the',

Well, the English is certainly native level and they appear to make none of the common mistakes second language learners do, like forgetting or misusing 'the', but that doesn't mean Satoshi isn't ethnically Japanese. Frankly, for all we know, Satoshi could be a woman. People raised in the Japanese culture express themselves very differently, though, and I'm not seeing that in this writing. This tends to survive, even when they're highly skilled in English, because they're not saying anything incorrectly, just differently. For example, one of my friends always refers to Earth as "our Earth" instead of "the Earth" (I rather like that one, personally).