We need to define terms.
Zoro is more lethal than Luffy, that doesn’t mean he’s outright stronger. It’s a lot easier to hurt someone with a blade than it is to hurt them with blunt force.
A 3 year old couldn’t very much hurt you by punching you, but she could send you to the hospital with a kitchen knife.
Zoro is more lethal than Luffy, if Zoro actually cuts Kaido’s head off, then the only way Luffy could prove that he is as lethal as Zoro would be to punch Kaido’s head off lol.
But that doesn’t mean Zoro packs Luffy’s brute strength. Zoro isn’t batting Doflamingo across Dressrosa lol. At best Zoro just cuts him down.