Kaido knows and tells Luffy how zoan df users look like when they awaken their devil fruits. That does not mean he awakened it, it means that we know how a zoan awakened should look, and we have no panel of Kaido looking like one. Even if he's not awakened though it would be weird for Kaido not to know how awakened users look.
I'm not ruling out the idea of Kaido being awakened, but we have no confirmation by facts.
Kaido didn't have much difficult fights after Oden so he could grow more in strength, as far as we know. He was already a yonko after all, with this great haki - df - abilities (speed etc) he was already powerful enough.
Then there goes the matter of df difficulty of awakening. A leopard might be easy to master, but a dragon? Kaido of course should be powerful enough to do so, he was lacking a clear mind though, i don't know if that matters.