Kaido being an actual dragon makes more sense than a devil fruit otherwise, the statement of "Land Sea or Air" Makes no sense because devil fruit users can't be in water. Note that the 4 Dragon Kings are actually rulers of different oceans and make their domain there. So, Kaido being able to swim is necessary for that statement to hold any credence (even if you don't believe it)
It seems @Jo_Ndule may be right about the possibility of Kaido simply being the Azure Dragon but having the ability to shapeshift in any forms.
However, if Dragon is just a dragon, forms such as zoan hybrid or even zoan awakening are not possible for Kaido?