If they decline to sell, then no debt exists in the first place, regardless of what forms of tender may be offered. If they had already sold it to the guy, and it was time to complete the transaction, then they would have to accept cash. E.g., you eat a meal in a restaurant, and the waiter brings the check. A debt now exists, and you can pay in cash, without regard to the restaurant's wishes.
The way I read the article is that when the fellow went to pay he used a credit card and when it showed up on his system that he had reached his purchase limit, he was refused the sale. What I wanted to know, was what would have happened if he had payed with cash and didn't or claimed he didn't have a credit card and couldn't or wouldn't provide any other form of ID. Example: He walks into the store, plunks down $500 bucks and says give me an Ipad and refuses to provide any more information. Would he be refused a sale even though he had "cash in hand"? And if the answer is yes, it would seem to open Apple up to all kinds of claims from discrimination to false advertising.