I don't think it was ever registered a sack. Damn Dobbs chucked it away while being spun around like some other QBs have in the past vs Donald. They would call in the grasp or grounding if anything but since he let it fly and they didn't clarify then AD doesn't get a true sack. I know... I'm pissed about it too. I would like the stat guys to credit him with one. I think it's a cheap way for QBs to get out of it. Dude got taken down and threw a prayer with no hope of completing just to avoid a sack.
Edit: okay TFL, I still think TFL on a QB should be a sack but these rules are always changing.