They looked only at fast food, buffet and snack/cafe places:

  The Spend data covers major national brands disproportionately, making it much more representative for limited-service/fast-food restaurants than full-service restaurants. We therefore restrict the sample to POIs reporting NAICS codes corresponding to limited-service eating places (NAICS 722513, 722514, and 722515) and associated with a brand for which there is non-missing visit  and spend data. This encompasses three subcategories: limited-service  restaurants (e.g., Taco Bell, McDonalds); cafeterias, grill buffets, and buffets (e.g., Hartz Chicken Buffet); and snack and nonalcoholic beverage bars (e.g., Starbucks). Our final sample contains over 100,000 unique restaurants from  over 600 distinct brands. From January 2019 to December 2022, our sample captures a total of about $24 billion in nominal sales.