I’m pretty sure it’s in the original Dramatica theory book, it’s been like that since ‘95. The searches you’re doing are gathering up the totality of discussion on the matter, and most of that discussion revolves around looking at the Main Character Throughline to help assess Story Judgment–because that’s typically the easiest way.
A quick search of the discussions here for story judgment audience bring up this treasure from 2017:
We’ll look into how we can make this more apparent to Narrova going forward.
The searches you’re doing are gathering up the totality of discussion on the matter, and most of that discussion revolves around looking at the Main Character Throughline to help assess Story Judgment–because that’s typically the easiest way.