Scale defence is a hard counter to non Ryou infused attacks. It's like saying can Superman damage Enel without Haki or rubber gloves.
Luffy was more satisfied with his base tree destroying fist being effective against Kaido than the Kong organ he used.
Now Luffy has the best Ryou on the island.
That example has nothing to do with Kaido's scales. His scales are a solid thing which you can touch and damage when you have the enough damage, like any other material or item, there is nothing special about them. You need enough amount of AP, that's all.
adv haki bypasses durability, regular haki doesn't, it does not matter AP level if you have adv haki, even chopper could damage kaido if he had ryo.
Adv Haki does not bypass durability.
Emision Haki is normal Haki launched at a distance (and it is more powerfull than normal Haki).
Penetration Haki is the Emision one but it attacks within an object, but that part of the object still has some durability which the attack needs to surpass if it wants to damage it.
And scabbards does not have Penetration Haki, nor Oden. There is no proof of that.
I think of it like this:
If you hit a logia user with a hakiless King Kong Gun, the user won't take damage, but if you hit it with a normal punch with armament haki the user will take damage.
That's the thing about Kaido, if you hit him with a non ryo King Kong Gun he won't take damage and yet the Scabbards that don't have attacks on the level of KKG can hurt Kaido because they have ryou
No, wrong. That has nothing to do with Kaido's body. He did not have those features. There is nothing special about his hardness, he simply has X durability/hardness and you need that amount of power to damage him.
Things that ignore durability are things like Amputation of Law within his Room, Perona's ghosts, Sugar's ability, etc.
Advanced Haki is only another amplification of the power with Emission and Penetration still need to surpass the inside of the objetc/material/skin/whatever if wants to break it. The durability inside those things is not 0 (ZERO), it has X.
And anyway, scabbards does not have Penetration Haki. There is no hint of that cause Oden has no hint of that Haki either.