The (female) author's point isn't that "Wal-Mart can't do what Costco does because it has more SKUs and thus a higher density of employees." It's that Wal-Mart,

The (female) author's point isn't that "Wal-Mart can't do what Costco does because it has more SKUs and thus a higher density of employees." It's that Wal-Mart, Costco, and TJ's have so many differences on so many axes that it is quite silly to make naive comparisons between their level of employee compensation while ignoring everything else that makes them different. I find this example particularly baffling, as one visit to TJ's should be all one needs to recognize that it's not even in the same category as a store like Wal-Mart.