Having started about the business of making tea, Niou's head is turned away from them when Azura voices the missing piece to this puzzle. It hides the flash in his eyes. So, the princess did covertly do something to Yukimura. The magic he threw at her was self-defense.

He resists his first impulse to assert that she won't attack at all, because he wasn't going to let her. Resists the urge to get between them with his knife set to her throat so fast she'd think it a teleportation spell of his own. Proving that the preemptive strike was hers would be extremely difficult, and Yukimura was trying to smooth tensions, not escalate them. He didn't want a war with Nohr.

More to the point, what was her objective? If she meant to assassinate the king, she wouldn't give herself away like that, would she? No. Her eyes hadn't quite been on the king. They'd been on--

...

He moves to the cabinet and retrieves the matcha, sifter, whisk, kettle, and three sipping bowls. He sifts the matcha into the bowls, then conjures water into the kettle. As he pulls his fire tome to boil the water, he removes a small packet from his hidden arsenal of tricks. The fine powder inside it joins one of the bowls.

"Be just a minute. Go ahead and have a seat."