Again this is surface-level as hell. First of all, this would be Oda showing us a unique situation, we'd have no real proof that Zoro couldn't do killing-damage to healthy Kaido, just that he didn't in this situation and it'd be ambiguous as to how Zoro would do if Luffy hadn't already beaten him.
Regardless, the point isn't to make Zoro God or Sword God-level in strength, it's to validate the idea that Zoro has more to give to the story and the OP world than just being strong; he'd be revered and remembered as a national treasure of Wano, a figure inspiring safety and relief to the people of Wano. Putting that finality down is important after 20 years of suffering, it gives Zoro's typically 1-dimensional strength more depth. Oda needs to give Zoro a unique importance as his role in the crew isn't often that necessary. Being the only swordsman, only he can kill Kaido. Everyone has their role in the crew, this is Zoro's.