To be fair, he DID know leia was lying about being on a diplomatic mission.
Well, he tells the officer that he traced the plans to her. They suspected she had the plans. "Several transmissions were beamed to this ship by rebel spies." Which again, is kinda true, lol. If by "beamed", he meant "Carried on board from the ship they were actually beamed to while I murderized everyone with my lightsaber."
Accusing her of lying about a diplomatic mission may have just been calling her bluff. It could also be that she's at Tatooine, and not at Alderaan, lol. "Yeah, Princess. A diplomatic mission to Alderaan. Taking the scenic route I see."