Not convinced Sanji's blue flames are hotter than Kaido/King's magma like flames.
And I don't think any character in series has portrayed their heat in such a superlative manner.
I don't really get why the fandom randomly rolled with Sanji having the hottest flames TBH.
If it was true, I'd expect that to be mentioned in series.
Did Shanks not low diff Kidd because he used Haoshoku coating?
And it's not clear yet that Zoro has been pushed to use KOH.
It seems like he's only hardening his swords in the last panel, and the tiny haki tendrils we see just looks like excessive COA:
Zoro hasn't even attacked Lucci yet with his overflowing COA, and whether he uses Haoshoku coating or not is still up in the air.
It's not King of Hell unless Zoro says "King of Hell".