If you read carefully, you'll notice that the article never actually claims there is a causal relationship between the ad and the share price dropping ("price dropped because of X"), just a temporal one ("X happened, and some time after that the price dropped"). This is a trick the business media pulls all the time. The reality is that they don't know why the share price changed, but reporting just on the change isn't very interesting, so they'll pick some plausible reason and run with it.