It's crazy that we're this far past Wano and some still don't understand KoH or enma.
King of Hell (KoH) is a style or technique where Zoro releases or uses overwhelming amounts of CoA haki, that is represented by smoke coming from the sword(s), combined with ACoC or CoC coating.
It is called KoH because Zoro's inspiration for the style comes from when Zoro released all of his haki to fully control and master enma for the first time. It is not because enma is needed to use KoH.
Zoro doesn't need overwhelming CoA haki to control enma. All he needs is large amounts of CoA haki which is different than overwhelming amounts of CoA haki.
In the above scene, Zoro is easily controlling enma without using KoH, CoC or even overwhelming amounts of CoA haki.
KoH can be added to any number of swords, technique or style. KoH is not determined by the number of swords Zoro is using.
In the bottom right scene we see Zoro say, KoH and then 3 sword style.
Zoro can use overwhelming amounts of CoA haki separately from ACoC.
In the above right scene, we see Zoro only releasing overwhelming amounts of CoA haki. We don't see any ACoC until the bottom right scene where he combines it with his overwhelming amount of CoA haki to create a KoH attack.
In the bottom scene, we see Zoro used ACoC without uses overwhelming amounts of CoA haki. Zoro doesn't call the attack a KoH attack.
Zoro uses ACoC with his demon splitter.
Zoro doesn't release all of his ACoC with KoH. Only when Zoro is using Dragon or serpent style does he release max CoC output.
Enma doesn't give Zoro more haki, boost his haki, Oden haki or CoC haki. All enma does is force the user to release more of their haki or ryu if they can't fully control the sword.
All of that was explained when Zoro used enma the first time.
That has been consistent throughout the entire manga. Every single time Zoro lost control of enma he only see CoA haki being forced out. We don't see CoC haki or black lightning.
What is also consistent is the smoke coming off the sword which means an
overwhelming or enormous amount of CoA haki was added to the sword. All those scenes proves that.
Plus, not a single time that did we see enma having any effect on Zoro's other swords. If enma is the reason Zoro can use KoH then how does he use it with his other KoH? Answer, it is not the reason.
Ryu is Wano term for CoA haki only. It is not CoC haki or CoO. When Hyou was training Luffy ryu/CoA haki and how to flow his haki we only saw examples of CoA haki. Asura called Oden's CoC a special trick.
Zoro not knowing he possesses CoC doesn't mean anything. Luffy didn't know he possessed CoC until he released CoC that knocked out the guards he was yelling at to stop. The manga tells us haki blossoms the most in battle.
Luffy proved the skills needed to flow CoC from the body is the exact same skills used to flow CoA from the body. Since Zoro was already incredibly skilled at flowing CoA it made it very easy for him to learn how to flow and control CoC after he unlocked it.