Deep in the text they mention that a study looked at nutrient contents and Walmart was the best:

“I learned this from talking to Brent Overcash, co-founder of a startup called TeakOrigin, which specialized in testing nutrient content in groceries from retail grocery stores. For years, every week, his team would walk into grocery stores, buy thousands of produce items the way normal consumers would, and bring them back to the lab to assess nutrient content.“