From the description of the study, it would appear that the only thing considered was the increase in fast food workers' wages to $15/hour.

That is a methodological flaw that ignores a huge impact. The wages for all their suppliers would also increase to a minimum of $15/hour. Their meat would cost more, their bread would cost more, their lettuce would cost more, their equipment would cost more, you name it would cost more. When all those were rolled in, the costs would probably approximate Norway. A Whopper at Burger King in Stavanger is $11, and a burger in a full-service restaurant is $28.