I think when Kaido said that, because it's kinda true, Roger who was PK never had a DF but only mastered the 3 types of haki and was the most powerful swordsman in his time, and when he said that, because he was kinda right about a DF power only won't rule the world but at the same time he was wrong cause there are broken DF's like Uta, Sugar, Baccarat (Non-Canon), Ain (Non-Canon)...etc, and other powers that can rule the world such as Ancient Weapons as Shirahoshi has Poseidon and if she can have full control of it she can rule the seas.
Also when Kaido said that, he was heavily utilizing both of his DF powers and Haki as well, and when he said this to Luffy, reason why he said that is because we kinda shouldn't take what he said seriously that much, cause he was fighting Luffy, and you know, when you fight your enemy, you use trash talk to play mental warfare games with your opponent, Kaido was playing mind games with Luffy when he said that.
Thoughts?
Also when Kaido said that, he was heavily utilizing both of his DF powers and Haki as well, and when he said this to Luffy, reason why he said that is because we kinda shouldn't take what he said seriously that much, cause he was fighting Luffy, and you know, when you fight your enemy, you use trash talk to play mental warfare games with your opponent, Kaido was playing mind games with Luffy when he said that.
Thoughts?