The legend of Demon Saint had actually already been around for a long time, but as Old Antique had said, this was knowledge that students would only learn about in advanced textbooks at university; most students in high school wouldn't come across it.
Demon Saint did exist. According to Old Antique, General Yi and the other nine Founding Generals had concocted a plan to capture demon gods alive, which was far harder to do than just directly killing them. They had learnt about Demon Saint from the three demon gods that they had captured alive, but not long after their memories were extracted, the skins of these three demon gods had started to disintegrate before everyone's eyes, until they were finally reduced to flying ash.
"Did those demon gods die?" Listening to Old Antique's story, everyone in class was a little confused.