I believe the technique the Red Scabbards are using against Kaido is similar to Enma: the difference is that Enma steal the User’s Haki by force and needs to be tamed.
We all know Haki can be imbued into weapons however the general belief is that when the User does that, Haki returns to him once he is done.
It’s like sword simply functions as an additional limb or body part.
But what if the swords don’t return the Haki they consume from the User and start to have their own Haki/Energy? Oda teased this so much!
In Loguetown, when Zoro bought 2 new swords, the shop owner/Tashigi explained Cursed Swords, they described them as independent beings who change based on their Users (previous ones). And we had many moments where Zoro felt the presence of Swords such as when Chopper came to his rescue in Water 7.
Where I’m trying to get here is that the Swords not only can be imbued with Haki, but they can be fed Haki to remain inside. What we are seeing here is 20 Years of Scabbards Will/Haki/Energy fed into their Swords!
If we talk about how strong is their Haki the moment they release it then it’s not that impressive, it’s not that unique, even Characters like Vergo could have greater Haki, however imagine 20 years of feeding your sword with Haki and look how effective it is against Strongest Creature.
*Theory by Salah Eddine