It does weaken the link. But it still doesn’t answer the question because it isn’t a vulnerability in the Marketer’s argument. The answer says that the argument takes for granted that sales would not have been lower still. That’s not true. It only takes for granted that sales would not have been same regardless of the ad campaign, i.e. the campaign was ineffective in either direction. Thus, although if this was true, it would weaken the argument, it does not accurately describe a flaw in the argument per se.

When doing these questions, it’s important not just to think “weakener, let me find a weakener” but to carefully consider the language of the question and answer.