The noodles are the entire bowl. 220g of cooked buckwheat and starch noodles is about 330 calories, and there is very little else: 25g of beef brisket, half an egg, some radish and pear. That is why naengmyeon feels light but still carries 90g of carbs. It is a plate of noodles in cold water.
The broth is sweeter than it tastes. Restaurant naengmyeon broth is beef stock or dongchimi juice with sugar and vinegar, and 400ml of it is about 60 calories with 10g of sugar. The habit of adding more sugar and vinegar at the table can double that. Still, compared with a 690 calorie bowl of jjamppong, naengmyeon broth is a rounding error.
The problem is when you eat it. A bowl of naengmyeon at 11,000 to 16,000 won is usually not lunch. It is the finisher after 2 servings of galbi or samgyeopsal, which means adding 480 calories to a meal that was already 1,200. The 사리 (extra noodles) for 3,000 won is another 300 on top.