That both Kimblee and Fool Bright are telling the truth, and that the note was written by a demon in order to pit them against each other.

Think about it. Why would they put one of their own on the line in such a way? Naming Fool Bright as an innocent has put him under considerable scrutiny and in very real danger. It would have been much simpler and less risky to choose someone at random. The demons know who isn't a demon, after all, and while they might be taking a risk of choosing one of the Sinners, the odds would be highly in their favor that whomever they chose to clear would come up clear, if investigated.

And in fact, by choosing someone who was so vocally and obstinately opposed to Kimblee's claim, they've created a win-win situation. If the group falls for it and votes out Kimblee, we lose our last Seer. If we instead turn our suspicions on the detective, we execute an innocent and subsequently become extremely mistrustful toward Kimblee.