A television advertisement shows an actor taking an antihistamine and then joyously walking though a grassy, flower-filled meadow without experiencing the usual allergy symptoms.

What, if anything, does the ad prove about alleviation of allergies?

Nothing

Explanation:

A television ad is designed to make you want to buy something.

The actor may have been filmed in a studio in front of a green screen with the grassy, flower-filled meadow added afterward, or they may not even be allergic to pollen.

Even if the scene is legitimate, the use of one subject proves nothing. You need a number of subjects in a carefully designed experiment to make a statistically valid conclusion.