They define "food desert" as 10 mile distance. I don't know how many of those 23 mil. don't have any means of transportation, but most probably do, in which cases 10 mile is just 10-20 minutes of driving away.
>In 2010, the United States Department of Agriculture (USDA) reported that 23.5 million Americans live in "food deserts", meaning that they live more than one mile from a supermarket in urban or suburban areas and more than 10 miles from a supermarket in rural areas.[6]