He's referring to the cost to the store to maintain at least one staff member at a time. That would be at least 3 different staff members in 8 hour shifts. Of course you'd have to take into account lunches, breaks, overlap, ext. And I'd assume the have a couple of days off.
He's simplifying it to the cost to maintain staff in the store. The cost per worker can vary, base on whether they're full time, part time, ECT. Whether they receive health care through work, the additional costs (workman's comp, taxes that the employer pays, etc.)
He is trying to provide an easy to calculate cost to demonstrate how expensive the wage, on its own, is to a business. That's the lowest cost a 24/7 business with only one staff member at a time could expect to pay, assuming we ignore the complications of scheduling breaks and other overlap.