It's not just to parallel Ryuma. When Zoro first came to Wano, he was scorned as a murderer and grave robber:
Someone who stole Wano's national treasure and caused misfortune to befall the country as they incurred the wrath of the Sword God:
Even the person most sympathetic to Zoro (Hiyori) insisted that he return their treasure:
Stressing that it's revered as a relic:
He is going to visit Ryuma's grave after the end of the war:
Before that visit, he needs to be acknowledged as a hero to the people of Wano. He
must be vindicated. Ryuma's soul and Shusui itself personally chose Zoro after all:
Saving Wano in its most dire moment and coincidentally replicating Ryuma's legend does that.
Defeating King does not do that. None of the people in the Flower Capital care about King. King is not the source of their torment and oppression. He was never a threat to them.
ZKK isn't necessary to get a Ryuma parallel. It doesn't need to happen for the fanservice of Zoro cutting a dragon. It's actually important for Zoro's character that he's vindicated in this way to the people of Wanokuni.
This is what I mean by: "you don't even understand ZKK". Your complaints don't actually engage with the theory. You're refuting and debunking a caricature of the theory, not the actual theory.
Here's a thorough explanation of the Ryuma argument for ZKK.