But marco has no permanent scars unlike Kaido. Marco lost to blaackbeard, but he walked out with no scars yet Izo got his face scarred
I think this is just Durability for Kaido. Like Its the equivalent of getting a paper cut.
There's times i've gotten paper cuts and not even felt it. Like I can get injured and then hours passs and when itake a shower, the soap stings the wound and thats when i realise i was injured and then by the next morning, It's like nothing happened. No scar, or anything. I don't consider myself having a significant healing factor, its just that the paper cut is too small for my Durability.
I think thats what's happening with Kaido here. First of all you cant hurt him at all without the special haki, and then when you get the special haki, Commander level AP still registers like just a paper cut for Kaido. It looks like Denjiro stabbing his whole sword into kaido's chest should matter but its basically like getting stabbed with acupunture needles for kaido. It just looks worse than the actual damageits doing.
As a comparison to marco, MArco isnt bullet proof. If you get a regualr gun, and shoot marco, He will take human damage, but then instantly regenerate. While for kaido, he's bullet proof so he takes literally no damage so he doesn't have to regenerate. Although if you toss a nuclear bomb at marco, He'll get completely eviscerated but his power could bring him back entirely again with NO DAMAGE. And as for Kaido, he could possibly tank the nuke but he'll probably actually get burned significally. And that burn WILL NOT get healed away.
Kaido tanks the nuke but walks away with a significant burn that will never heal, whereas Marco would get completely eviscerated but he'd completely regenerate after some time and walk away with nor scars or burns.