There is really no two ways about this, if you eat less, the excess weight you have on your body will be used up and you will become slim. The professor did go too far in terms of eating unhealthy and I think anyone trying to lose weight should pay attention to both the quantity and the quality of foods they eat.
It is more important to reduce calories consumed than to burn calories through exercise when someone is trying to lose weight. Eating fewer calories is the best method of losing weight. If the food consumed is unhealthy, it can cause other health problems, but the individual will still lose weight.
People bought the argument about fat because the sugar lobby convinced researchers to find that fat was more harmful than sugar. The body isn't that hard to understand. If you take in more than you burn, you gain weight. If you take in less than you burn, you lose weight.
While calories may be more influential on weight loss, fat content is also very important for someone who wants to lose weight to improve health. We also don't know what other things the professor was doing. Perhaps he was working out obsessively. Calories in versus calories out is key in losing weight, but it's also important to eat healthy, nutritious foods that offer the body nutrients. Losing weight doesn't matter if you have a heart attack at 40 because you consumed too much fat.