Zoro isn't being setup to reopen Kaido's scar, he's been setup to slay him.
The feat the scabbards were incapable of, that only Oden could do was killing Kaido. That's the bar for a "monster samurai" of the likes of Kozuki Oden.
Kaido laments that the samurai can't kill him, belittling the samurai as being too weak to even open his scar:
He comments that he saw the visage of Oden and could have let them kill him, but that they aren't Oden. He then proclaims that there will never again be a samurai of Oden's calibre:
The feat Kaido was ascribing to Oden was killing him, not merely reopening his scar. If Zoro is to be recognised as a Samurai on par with Oden, then he would have the ability to kill Kaido.
Recall that Zoro is being setup to surpass not match Oden. He'll do that which Oden left unfinished (defeat Kaido).
The "Second Ryuma" and dragon slaying foreshadowing further support this.