No, the 'calorie is a calorie' theory is outdated. 1500 calories a day of organic vegetables, fruits, legumes, nuts, gluten-free whole grains, etc is NOT the same as eating 1500 calories a day of processed burgers, fries, sugary cereals, artificially-sweetened beverages. The former is healing for the body while the latter causes significant inflammation and degeneration to the body. Just because you can stay slim eating reduced calorie doesn't really tell us anything about what is going on inside our bodies with the kinds of food that we eat. Furthermore, people who diet by eating low quality crap food are not giving their bodies adequate nutrition, and I'd surmise that those people would be more likely to regain their weight after they stop being able to white-knuckle their way through reduced portions, and go back to old ways of eating larger portions. Giving good wholesome unprocessed food to your body is the way to nourish yourself and actually be healthy.... not by eating McDonalds, at least not the McDonald's food that I see people eat.
Additionally, fit doesn't necessarily mean healthy. Of course, being excessively overweight is definitely unhealthy. (This is not an aesthetic issue, nor any issue to discriminate a person on, but we do know that fat, especially visceral fat, causes significant inflammation and insulin resistance.) But just because someone is in shape, if they are slim by eating portion-controlled crap food (like the diet sodas, processed food etc), that is definitely not healthy. When a person is young, he/she can still look good externally if he/she is in shape, even if he/she eats crap. However, that kind of garbage food is still aging a person internally and causing degeneration, cancer, atherosclerosis etc.