You seem to have missed the point of the question "After all, how do you know that sales wouldn't be down 14% without the ads?".

The commercial is not the only thing affecting sales. If there is some other factor driving sales down strongly the sales may still fall overall even if the commercial is successful (i.e. attracted a large number of sales).

If sales would have been -14% without the commercial but are only -7% with the commercial, then the commercial did raise sales. You measure vs. the counterfactual baseline, the one where the rest of the world is held constant but you don't take the action that you're measuring.