Ashura is explained as Manifestation of his conqueror's Haki.
1. Cannot be trained
2. You have to be born with lol
And this is the ability that scarred Kaido, therefore by definition proving that Zoro’s best feat in One Piece was something he achieved because of his genetic lineage and not training.
Zoro's greatest mode is just part of his genetics
If Zoro wasn’t born as Ryuma’s descendant, he would be no different from Kamazou right now: competent but forever destined to be the bitch of someone born greater than him.