They calculated the calorific consumption of America's population based on data in the National Health and Nutrition Examination Survey carried out by the country's Centres for Disease Control and Prevention. They then compared that figure with recorded levels of food production, modified for imports and exports. They found that the average American wastes 1,400 kilocalories a day.
They assume that all food produced, but not consumed by humans, is wasted. I read the study, and it did not mention anything about excluding food that is meant for pets. I am almost certain that their measures of food production include everything sold by farmers, including agricultural products that end up in pet food. Even if they do exclude pet food, many people feed their animals food that is meant for humans.
A medium-sized dog (30 kilograms) consumes about 1500 calories a day. This may be considered waste by many people, but it is not a simple story of Americans throwing food into the trash.