And multiply the Haki's user by several folds..
Of course you would say that you see a Huge Haki feat, so Zoro has to do it too..
What? When do weapons multiple the users haki?
I talked about black blades being able handled unlimited amounts haki channeled through them months ago.
If you were paying attention to the manga you would have realized Oda has set up Zoro to use huge amounts of haki for a while. Zoro main power is channeling power through his swords. It went from only physical strength to mostly strength to mostly haki.
During Dressrosa Zoro could channel incredible physical strength through his swords to create attacks on BM Elbaf Spear level.
During Wano war Zoro channeled large amounts of powerful CoA haki.
Against King, Zoro channeled overwhelming haki and flames through his swords and combined it with ACoC to create KoH.
Zoro defeated King by channeling overwhelming CoA haki, overwhelming ACoC haki and flames that creates KoH a Dragon sword style techniques.