Zoro had Kaido's kryptonite, Enma, and Zoro is one of the guys from the new generation who has been said twice as the only ones capable of defeating Kaido.
How come Dr. Xebec, with a Haki far superior to Zoro's and in perfect physical condition, can't leave any scars on Kaido, man?
A 21-year-old Kaido who is less resilient, I don't think he even knew how to coat himself in Haki.
Let's make the situation worse: how is it that the guy with the strongest Haki of the old generation, capable of splitting the skies single-handedly, with Haki superior to Roger's and Garp's, using a sword, stabs Kaido with that same Haki, and that attack isn't even deep enough to leave a scar on 21-year-old Kaido, who couldn't even (probably) coat himself in Acoc Haki?