I know that this is official number, but nobody really cares if it was 3, 4, 6 or 2 millions. It changes nothing.The number of casualties in Auschwitz is claimed one, while officially stated another, for starters. I don't deny something happened, but I am skeptical that it's exactly six million, the same number claimed in publications dating back 50 years before that.
But what I have noticed is that Holocaust deniers usually start with how 6 million actually didn't die, then eventually they start talking about how not that many of them died at all and eventually we reach a point where they start talking about how Holocaust actually didn't happen at all.
As for Vox, I think at some point he had a theory that Holcaust happened, but it was actually done by Soviets.