She's antagonistic toward a person who has lied to all of the leaders of her home, after they had all worked to bring him to their world to save them from an impending calamity.
He didn't. Lie I mean. Not really. They used a hell summoning spell, ergo he's a destined hero. Or it's their fault they screwed up and they just kidnapped a random kid with a sword. He might not feel very hero-like or confident in himself, but who knows what qualifies one as "destined hero".
Now, yes, the Princess should know this, but she probably is deciding that she needs whatever method to wrangle the hero to actually try, at least if the situation really is that desperate for them that they are willing to go for "summon random hero from another world and hope he's neither incompetent or an asshole" strategy (They didn't slap him with a magical slavery collar or fed him an obviously bull story about how every other race is terrible and needed to be genocided, so they are probably not the "evil kingdom doing hero summoning as SOP" type).