Well the manga has made it it clear that ONLY ODEN has proven Kaido isn't invulnerable. For 40 yards of piracy, it didn't matter whether Kaido was a Rookie fighting the fuckin pirate king and Garp or he was a full blown Yonkou confronting shanks dung marineford, ONLY ODEN scarred him. Haki or no haki, zoan form or no zoan form. ONLY MOTHERFUCKIN ODEN
He never got anymore scars because he wouldnt be dumb enough to fight them Zoan form or be distracted. Top Tiers could obviously scar him if they got Oden's chance.
And you realise we've seen people use Haki WHILE UNCONSCIOUS. Like Luffy knocked out fodder while unconscious and Whitebeard defaeted rookie Ace while he was asleep. So high tier Luffy and top tier Whitbeard can use Haki while they are unconscious but a fully conscious Kaido in Zoan form for some reason Cannot use haki