No supermarkets can enforce it, nor should it be their job to police it. Some people are exempt after all. Sainsbury's, Asda, Morrisons etc, all of their policies are the same. Basically please do wear a covering if you can but we won't be questioning you. Quite right too I think, to protect those who are exempt from the awkwardness. Nobody should be forced to publicly state their personal medical conditions in order to justify the absence of a face covering. Stores are simply trusting that those who don't, can't.
That said, I'm surprised in current times they wish to run an add when they do not promote the use of a face covering. Even if it was filmed before the rule came in it's been known about for quite a while.