You think teenage king can beat doffy?
They are two different cases, from the point of view of the kaido, someone so young, enduring what the king endured is worthy of bravery and praise, the young king is strong when compared to others of his age, there is a semantic gap for interpretations that the KAIDO's speech in relation to the KING gives us.
Also, one thing has nothing to do with the other, whoever declared Kaido as the strongest was KING and not Kaido himself, you can't take Kaido's statements seriously, that's one thing, but don't take it seriously the statements of the other characters, it makes no sense.