Zoro clearly said "he's fast therefore I can cut him now", while saying earlier "his flames are on, no point attacking him now"
Perhaps because he opened a wound on King has blocking it as his chest part was bleeding a lot, Zoto couldn't cut his leg which he blocked one of Zoro's strikes in that exchange, one can assume he was guarding his wounded area.
Otherwise if he could clearly cut a flame on King Oda would need to clearly show it, but every instance he was cut his flame behind his back was off. After King got wounded in his chest he was with flame mode while guarding with his other bodyparts like his legs and his sword.
Dude, he didn't know that he can damage King's defence mode until King himself started blocking.
How could he? King was no selling everything prior to that. And the only info he had then was regarding King's defence/speed mode mechanics.
While King likely knows, cause of experience with an advanced conqueror's user in Kaido.
Let's be intellectually honest here.
What is the purpose of that panel where King was blocking, on backfoot, and Zoro taunting him? It is Oda narratively spelling it out that King can no longer no-sell Zoro's attacks in defence mode.
Clearly, KoH Zoro can hurt defence mode King. Or else, that page had zero purpose.
And like I said, we don't know if King was in defence mode in that final attack. Too much fire cluttering.
Precedence however suggests that he might have been in defence mode, since all of King's prior fire attacks have been in his in defence mode.