The study conclusively proves that it's not a good idea to starve yourself on 1500 calories/day of low-grade carbohydrates. Considering their diet was also devoid of vitamins, fats, and proteins, you can't just say: "a ha! A diet composed entirely of pasta, potatoes, and rutabagas gave bad results - therefore, diets don't work!" It comes across as more of a rationalization than anything. There's also a lot of evidence than eating a diet of nothing but empty calories is bad for you, even if you get in plenty of food.
It's easy to imagine that the subjects would have had much better results if they were allowed to eat some fruits or vegetables with their experiment. Or eggs, or dairy, or meat. The experiment really bore very little relation to the expected diet of a person who wanted to lose weight.