This is wrong. Chapter 1035 Zoro cut King even with his flames on. So beginner level conquerors coating is able to cut King with his flames on. Imagine conquerors coating that has been constantly used against the strongest character in one piece. It's gonna take King out in 1 hit
The first time Zoro cuts him (when King's mask falls off) we literally see a wide shot of King's back with no flames.
The second time Zoro literally says :
"He is fast, so this means I can cut him"
thus highlighting that he can't cut King's durable form.
And then there's the final clash where we can see both in 1035 and 1036 that King didn't have any flame on his back.
(probably because he used them to create the big flame dragon)
So, if Zoro truly managed to cut his durable form then I'd like you to give me the page and panel please, because I couldn't find it.