[pitpatagain]

My comment is not whether the article's claim re this particular market inefficiency is empirically correct, only that it doesn't logically simplify to "if larger stores couldn't get preferential deals, prices for consumers would be significantly higher." That depends on a lot of things.

Grocery suppliers are, eg, agricultural producers who need to offload whole crops of perishable goods. The industry is full of well known market failures, examples of monopsony buying power, etc, so an argument from an idealized market I don't find very persuasive.