It was one thing for him to acknowledge that Mukuro was stronger than him. It was one thing for the Baby to acknowledge (and use) it. It was another when worthless weaklings decided to try their hand at insulting with it.
Nata may have said he was stating fact, but the difference changed nothing for Hibari - he was following swiftly, catch up in a matter of seconds, stride purposeful and direct. Cutting, almost; his face would never show it, but he was irked by this revelation. Some thing were best left unsaid outloud; and the Head Prefect's weaknesses were one of those. "He is," Kyouya said, easy, light, nearly gentle, and then, "You are not.
"You should learn to hold your tongue on facts, or else I'll have to bite it out." See? He could be nice. He was giving the herbivore a chance to take back his words. This was interesting (somewhat; only in the way he hadn't yet clocked the other), but it was no longer funny.