How do we know that though? Zoro has an inferior level of Haki than Luffy i.e., Luffy has ID while Zoro has the barrier variant. Your attacks are more effective on Kaido the higher your Haki level is. Luffy has an element of lethality to his attacks too because he targets the internals. Yet it was Zoro that was hurting Kaido better, meaning his AP is actually higher.
OKG was treated like a second-grade attack when compared to how DMG was treated. We have little to no reason to scale it as stronger b/w the two.