It doesn’t say that prices have to higher, food suppliers could meet the requirements by providing the same low prices they provide to big businesses to small retailers.
as the article states the problem is that the big supermarket chains dictate prices to their suppliers. that is, the suppliers can't afford to lower prices for everyone, but they are forced to to lower prices for the big chains because they also can't afford to lose the sales from the large chains. germany has the same problem.
what needs to stop is the abuse of the dominant position of the large chains.