Because you made it sound like his base didn't get any boost from his DF.
You are confusing things, there are traces of your zoan in its basic form, but that does not mean that all the durability you have obtained is hers.
Kaido has no scales in the basic form, he has durable skin and muscles that is originally "probably" due to his race.
And if you're talking about the flames (which levitated ONIGASHIMA), it's a skill of your DF, it doesn't change the fact that to execute it, he used the basic form.
Yes, he receives impulse from his DF, the accelerated recovery to the point of being considered a cure is one of the impulses of the DF, it acts automatically.
Now impulse, it's your hybrid form.
Increases strength, durability and others. He delivered the sheaths, with their basic strength, basic durability, if in his hybrid he probably has scales all over his body.