In a lot of regions metal is the official theme music of white nationalists so I can't say this is true

Eh, it doesn't belong to them. Racists try to insert themselves into things the same way other groups do, e.g. "christian rock" that tries to use the appeal of the genre to spread their particular message.

As a counterpoint to your statement, metal is immensely popular across South America. Look up any Iron Maiden concert in Brazil and you'll see vast legions of diehard fans going nuts. And the overwhelming majority probably wouldn't be considered white, at least not in the way European or North American racists want it to mean.