I dont know why you think that, i dont agree.
I dont remember to EVER see a Zoan be better than his base or hibrid form. Ever. And kaido was not using hibrid or base.
And besides that kaido as a dragon is huge and becomes a way more easy target for 9 skilled samurais at same time.
So...now with a way smaller body and most than probably stronger than his Dragon he Will look better.
Do you think their attacks will be ineffective on his smaller form which doesnt have dragon scales as they are ineffective on scaled large one?
Zoan is never better than base form indeed but that is in 1vs1 scenario, this is 9vs1 where the size of the body handles dmg better.
Yes, as a dragon he is a huge and easy target but the Scabbards dont have enough damage to harm him significantly.
Inuarashi has stuck him foot blade all the way into the dragon form and it did nothing to him, if he sticks it all the way in into base form, the blade will exit on the other side of Kaido... Surely, he is more agile in base form and has a club to defend with but it's still 9vs1.