Okay, so I'm the sort of person that doesn't trust the term "spicy" on recipes because in my experience, it almost never is. I stuck to the recipe more closely than I usually do. The only changes I made was that I used chili powder because I couldn't get gochugaru, and added a couple teaspoons more honey and mirin because I found the sauce slightly more bitter than I prefer. This stuff delivered. It was intensely spicy. Indeed, it was "too spicy", words I've not said in a long time.
I'm giving it a thumbs up anyway because wow, it delivers! Kudos to this recipe for putting the fear of God back in me.