Jjamppong looks like the light choice next to jajangmyeon because it is a soup. It is about 690 calories to jajangmyeon's 797, so yes, lighter, but only by 100. The noodles are identical: 250g of thick wheat noodles at about 360 calories. The difference is that the black bean sauce is fried in more oil than the chili broth.
The broth is not free. Restaurants stir-fry the cabbage, onion, squid and pork in a wok with 15g or more of chili-infused oil before adding stock, and that oil ends up as the red slick on top of the soup. Drink all the broth and you take in most of that 130 calories plus around 2,500mg of sodium, which is more than a full day's worth.
At 8,000 to 10,000 won a bowl it is a big lunch, and the bowls are large: 900g to 1,000g with broth. The lunch special that pairs it with a half plate of tangsuyuk pushes the meal past 1,300 calories. If you finish the noodles and leave half the broth, the bowl is closer to 620.