Okay but only Advanced CoA can hurt Kaido… this is demonstrated when Nothing Pre Kuri Luffy could do was hurting… and it was demonstrated when Kaido was VERY SURE the scabbards could not hurt him which would be extremely stupid for him to assume if regular CoA was all that was needed to hurt him
There is three levels to invisible Haki.
Basic - which is just coating yourself in invisible Haki. (Luffy was doing this against Caesar Clown)
Intermediate - Which is penetration.
Advanced - Which is internal Destruction.
What Zoro and the scabbards were using was penetration. So they were able to cut and penetrate Kaido's body, and hurt him.
Internal Destruction, which is the Advanced version, penetrates a persons body, then the haki explodes causing massive damage.
Zoro and the Scabbards do not have that, which is Advanced Invisible Haki. They had the second level, which is penetration. It's was still effective against Kaido, because they are Swordsmen, and swords damage a person differently. Since they could penetrate his body with their swords, naturally they could cut inside his body.