It really means that the cash payers are subsidizing the card payers, b/c the cash payers don't get the cake for cheaper.

I, as a cake seller have a cost i need to pay to the network every month, from my total revenue it's some percentage (depending on ratio of cash vs card). I obviously add that percentage to my cake prices to make up for that. Both cash and card payers pay the same amount for the cake, thus they share the price hike equally.

But the cost still finds its way to the end price of the product.