Sounds like the guy was eating fairly healthy for 2 out of the 3 meals and staying under 2000 calories a day. If you can eat 3 meals per day at McDonald's and stay under 2000 calories a day than you're obviously not getting the large value meal with fries and regular soda. At least in Supersize Me the guy actually ate what most of us would consider "typical" fast food meals (even if he was overeating to the point of physical discomfort.) The twinkie diet was similarly misleading because, other than the twinkies, the guy was eating fairly healthy.

"Mr Cisna embarked on the diet as an experiment to show how we can eat anything as long as we stick to a limit of 2,000 calories per day and stay close to the recommended dietary allowances for nutrients...On a typical day, Mr Cisna ate two egg white delights, a bowl of maple oatmeal and one percent milk for breakfast, and lunch would normally be a salad...But it wasn't just 'healthy' fast food that he ate; for dinner he would indulge in a value meal like a Big Mac or a quarter pounder with cheese, and he'd also sometimes snack on ice cream sundaes...During the three months, Mr Cisna - who admits that he didn't exercise or watch his calorie intake before the diet - also began walking for 45 minutes a day, a factor which no doubt contributed to the results. "