There is actually no explanation for the case Kaidou is dead.
In one piece Road To Laugh vol.2 or 3, the following description is given:
"Kaidou's scales and Haki have made him like an iron wall."
When Kaidou fell into the unconscious magma, what form was he in? In base form.
Can someone unconscious make use of your Haki? Not.
In basic form Kaido has the scales ? Not.
Zoro cut Kaido in dragon form in which his scales are stronger than the others and left a scar on Kaidou's hybrid form in which his scales are present + His Haki coating...
So I don't know why Akainu fans find the hypothetical situation of a Kaidou being killed while he was unconscious in his basic form, which is not even his most durable form,and that he wasn't even using Haki, if I were them I would be worried that Kaidou could easily withstand the temperature of Magma while coating himself in magma without taking damage from it.
Because it will be quite something if a Kaido with his natural resistance (Base) without making any use of Haki and his scales withstand the almighty magma he coated himself with without any problems while suffering 0 damage.