7.7% of census tracts, not of people. The overwhelming majority of those tracts have insufficient population to support a nearby supermarket. That doesn’t mean they don’t have access to food.
Most of these tracts are farming communities. They provide all the food stocked in these urban and suburban supermarkets. They are literally surrounded by food, in their fields, pastures, gardens, pantries, etc. But because the definition of “food deserts” focuses on supermarkets and doesn’t include the 10 tons of grain in their bin, they are considered to be living in a “food desert”.