A Yonko whose lived on Wano for over 20 years, doesn't understand that Haki is Ryuo, and the correlation between the two?
You do realize that when Luffy hit Kaido with Red Roc, he immediately saw Roger, Shanks, Oden, Whitebeard, and Rocks, right? Kaido understands full well that Haki can damage him, and who is capable of using the type of Haki that can damage him.