Zoro kill Kaido is still strong. King is just a tool Zoro used to help him gain the power needed to kill Kaido.
Even though King DF is a dragon while Kaido's DF us a fish. The citizens of Wano won't look at King think of dragon. Instead they would think of a dinosaur. The same with Kaido. They see him as a dragon not a fish.
If Zoro is to be the second Ryuma he needs to kill a dragon or what the citizens of Wano sees and believe is a dragon.