Why would he acknowledge zoro as a samurai greater than oden? Zoro isn’t a samurai, his ancestry be damned, the man grew up in East blue.
kaido knows that he isn’t from wano. Being a strong swordsman doesn’t automatically make him a samurai. Kaido knows of mihawk and shanks, swordsmen stronger than oden, he just doesn’t know of any one from wano of that level.
zoro isn’t from wano