What is a fiend anyway? That's basically just a term used by the villagers to call someone who has lost all death feedback and is deranged enough to kill people randomly. Or rather, it's not even such a scientific term but a part of their folklore, even though it's real, if rare. So, no matter how you look at it, a human who walks around killing other humans with cantus without succumbing to death feedback is tecnically a fiend. I reckon Shun-inside-of-Saki probably denied it because labelling someone a fiend is nothing but easy labelling. It's kind of like calling someone an enemy without trying to understand anything about the other side.

The preview especially suggests this was nothing but a trap. It's not necessarily true that Kiromaru and Squealer are honestly working together (assuming Squealer is even capable of working honestly with anyone), but it's looking more and more they queerats were after the bioweapon even more than humans now. Relying on a volatile fiend is hardly a perfect solution, in the end, and like I said earlier, I doubt it would work everywhere either.