I'd already seen your thread.
The answer lays in the execution. Garling was a cocky Mr. Nobody, who rushed Xebec, was one-shotted, and only dared approach Xebec again in group when this one was almost already dead.
Had Zoro done the same to Kaido, he'd been a rat, too, sure.
But I think the expectations for ZKK were different.
We would have had what is already canon: Zoro scarring Kaido, being shut out by him, then also blocking Kaido's and BM's hakai attack, before breaking all his bones. That's already leagues above everything Garling has shown in GV.
In ZKK, Kaido would have re-emerged from his hole under the volcano after being defeated by Luffy. He would have also been at 1 HP, and, just like on Thriller Bark with Kuma, since Luffy would've been passed out, it would have been Zoro's duty to protect the crew. It wouldn't have been a sneaky kill on a victim lying on the ground exhausted, but a forced move against a stubborn tyrant who won't accept defeat, not even with his dying breath. It would have been a last flying slash, a la Oden, being both a callback to the flashback, and a misery shot against yet another dragon terrorizing Wano for the third time in a row.
So, again, I don't think ZKK would've inherently made Zoro a rat, just as I don't think making Garling a rat in GV was Oda's only choice. He decided to depict it like that.