That means the thumb should be out of view, and the fingers should go from the back to behind the grip instead of the other way around. As shown in the picture linked above, the palm is facing out if you have your right hand behind your back. Of course this is not real life, but that's the only way we can more or less decide whether the statement is right, since we can't see behind Vault Boy's back. The hand in the picture would have to be with fingers facing us to be a right hand, but it's the thumb that's actually facing us, thus it's left hand.
If you REALLY want to argue details, it's impossible in real life to put your hand behind your back in such a manner as is show in the picture, see for a demonstration of how it looks. 'In the icon, it incorrectly shows the Vault Boy having two left hands.'
