This is awfully stupid. Their "proof" only relies on their own claim that the liquid only react to the poison and nothing else.
And the anachronistic modern spray bottle is the icing on the cake of lazy execution. In this medieval setting, a plastic tool with fine molding and precise mechanism would look like some alien artifact that would gather more interest than whatever they're talking about.
Edit: Wow, I didn't reach the point he pulled out his smartphone when I first commented. Nobody knows what a smartphone is, as far as they know, it's a magic box that talk, and that only the MC know how to operate, so they can just say he made it say whatever he wants by magic too.
