It's not; his forearm is twisted so that the back of the hand is facing outwards ( rather than the palm facing outwards). If you hold your arm up in that position you'll see the thumb will be pointing away from the body, not towards. Although granted, at the time that I drew this I could have made it a little more clear which way his forearm was positioned. It's harder to discern amidst the fire.
I did it, too, and no it doesn't. Just have your palm facing your face, and rotate your hand so your thumb is facing outwards just a little more. I've seen poses and reference photos like this. It's exaggerated a little, yes, but it's not that off.
4e D&D anyways. Prior to 4e, and still currently in Pathfinder D&D for instance, tieflings don't have any monolithic appearance. Lots of diversity depending on particular fiendish heritage, and just the random interaction of fiendish and mortal blood. Can't say I like how WotC handles tieflings in 4e, but Julie's art is still awesome.