You have to understand that off-panel, days long fights are nothing but hype, nobody lasts long on panel, especially not against lethal opponents like swordsmen. All it takes is one clean slice to end the fight, you cant expect Katakuri's 54-hit special from swordsmen...
Finalbeta is overdoing it, of course Zoro is not gonna make quick work of King, even if he could Oda would find a way to make the fight longer and troublesome for X reason. Many people didnt fight anyone and they are still up there, no reason why Zoro cant be, on contrary, there is no reason that doesnt allow him to be up there, he is a blank slate for Oda to do whatever he wants with him, he didnt write himself in a corner with Zoro because he didnt put any limits on him post TS.
Zoro's fights are hard for specific reasons, usually when he overcomes that reason he trashes the opponent and proceeds to fight more.
Extreme diff fights usually mean that you cant put any fight after them and I dont think that will be the case with King, where the only issue may be King's ability to use fire which may prevent Zoro from fighting at his best and push him into evasive maneuvers.