The walmartification description there applies to their suppliers, too. They offer a great purchase price (especially to food goods) and sell at a loss so both farmers and customers choose Walmart. Once competition is stifled, they slap the farmers with reduced offers (while no other chains have the total purchasing power anymore) while sale price can afford to come up a bit on the shelf.

This type of move by Huy Fong led to the Sriracha sauce shortage when the chili farmers rejected the low bid and were willing to let the crop rot.