AdCoC+Ashura could only scar Kaido, and didn't even drop him to his knees.
Kaido confirmed 1000 times more durable than King.
1 - Zoro didn't use ACoC against Kaido, only CoC. Kaido laughed at Luffy when he said he figured out Kaido used ACoC and implied he could use it.
2 - Zoro was injured and in a weaken state when he scarred Kaido so the strength of his attack was weaker.
3 - Even if you still believe Zoro used ACoC against Kaido it is illogical to believe subconsciously using so little ACoC black lightning is not visible is anywhere close in strength to consciously using so much ACoC it leaves a long trail of black lightning.
4 - King is more durable. Zoro stated the flames increases his defense, not durability. Internal destruction attacks don't hurt King/seraphims in flame that proves the flames don't increase durability since durability doesn't protect from internal destruction attacks..