I never said that he had to be American.  I just said that he probably is.  Most native English speakers are American.  I am having trouble finding numbers, but it is pretty clear that most native English speaking programmers are American.  Unless we have reason to believe that he isn't American, the parsimonious conclusion is that he probably is.

As for his demographics, let's be realistic.  This isn't TV.  Imagine that there was a black female programmer with the skills necessary to write the bitcoin client.  Do you really think that we wouldn't know about such a 1 in a trillion person?  She would be already be one of the most famous people in the world.  All of the elite universities would have fought over her.  Then every company in the world (and not just software companies) would fight fiercely to have her on their staff.  Imagine the Obama story x100.