Not only is One-Horn's plan the worst plan in the history of anything -- have a loud conversation in a hallway and hope someone important just happens to be both wandering by and so stupid that they would consider you a credible source of anything -- but also the world's stupidest demon happened to be standing nearby and totally fell for it.
This author belongs to the "I Can Do Anything Because This Is A Webtoon And I Have No Editor" school of storycrafting. And in this case, that's a bad thing.