You don't get it. It's not about scarring Kaido, it's about called him a significant amount of damage with an attack. Scarring it's just the results of a cutting attack causing Kaido a significant amount of damage. If Oda one of us to know that Luffy or Yamato caused Kakdo a significant amount of damage with their attacks that cause internal damage then he would have let us know. Just like he did with Doffy after he was hit with gamma knife.
So you can't use the "internal attacks don't leave external scars" excuse. Even when Zoro scared Kaido, Oda let know Zoro scarred. Unlike the Scabbards who combination attack couldn't even reopen up his old wound.
Zoro did block hakai.
Here are the scenes. Are you going to continue to act like these scenes don't exist?
True, he didn't block it for long but he still blocked it which gave everybody enough time to escape. If it wasn't for him all of the supernovas in the rooftop would be dead. That is an impressive feat and one of the greatest feats we have seen the entire manga. That power, barrier haki, can be used to block weaker smaller attacks. Barrier haki can be used offensively and defensively.
Who cares about knocking Kaido down. Luffy have knocked him down more than anybody and it has amounted to nothing. Zoro was not trying to knock him down but to take him down or take him out. Even though Zoro didn't he still caused Kaido a significant about the damage or scarring him. Which is morning anybody else is on. I've already showed you why that is. The crazy thing about that was since Zoro was severely injured and in poor condition that attack was not at full strength of that attack. Zoro is a strength based fighter and 20 to 30 broken bones limited to his strength. If you wasn't in a condition it would have been a much stronger attack I'm going to cause them more damage.
Kaido didn't ignore the damage. He even told Zoro scarred which is a feat achieve only by Oden. The benefit to that is Kaido now has a "chink in his armor" that can be taken advantage of.