There's a mistake here I believe. Whitebeard is the strongest pirate. The only difference is Kaido is the strongest creature ,i.e., Kaido has a body that cannot be injured easily to such a degree he's essentially immortal.
Whitebeard would die if he were stabbed, hung or put under the guillotine. Kaido on the other hand could be asleep and none of these would hurt him because his flesh is more or less made of steel in its effect.
It's not that Kaido is stronger than Whitebeard as a fighter, it's more of a case of Kaido being almost immortal due to his body make-up that he stands superior to Whitebeard.
Simply put, in the sense of who is physically closest to immortality, Kaido's superior to Whitebeard; this is not the same thing as being the weaker fighter.