This feels like a weaselly way of putting it. I want to know the average (or median) cost of cash handling, not what it costs for "many" (whatever that means) retailers. It could be that the 15.3% figure represents costs that 0.01% of retailers have to bear, and that 4.7% is for 10% of retailers. And it could be that 90% of retailers spend less than credit card fees managing cash.
Or they could mean what you want them to mean, and it really is more expensive than credit card fees for the median business to manage cash. But from their wording, it's hard to know the truth behind it.
Sniff-test-wise, I find it hard to believe that gas stations (and other retailers) would offer a cash discount if cash was actually more expensive for them to handle than credit cards. It seems unlikely that they wouldn't have come up with that without at least some research to quantify their cash-handling costs.