It sounds like they don't know the difference between gochujang and gochugaru and were thinking of the latter. Neither one is a standard ingredient in the dish, so it's hard to say. Japchae is just dangmyeon with stir fried mushroom and veggies and soy sauce, sesame oil, and sugar. Well, and a beef substitute, if you like. I don't usually bother.
Either one would add a nice kick.