The ball was never possessed by the defense, so there's no way it could be a touchback. In this case, once the ball went out of bounds the last team to possess it was Dallas. The ball is placed at the spot it left the field of play; hence it is a safety.
You're telling me what you think the rule should be. (And I agree.) But that's not what the rule is. The quotes I cited both would indicate that this was not a safety, and that this was a touchback. Show me anything (aside from reason) that would suggest that the play should not have been ruled a touchback.
ML/NJ