Oooh. An 8th prince is probably quite useful as a political hostage. Assuming neither Panamera nor the ballista kill him. I can sympathize with the urge to purge incompetent commanders too.
Though in his defense, I'm sure this prince is deeply confused and more than a little desperate. Like, strategically, the best move is obviously to either back off and report this gap in intel or defect to this superior power. But on a personal level, he's screwed if he goes back empty-handed, set up to fail, and common sense dictates that what he's seeing is impossible.
Rather than just arrogance (not that he's that smart anyway), I think betting on the assumption that you're looking at a bluff isn't necessarily wrong.