I don't understand this argument, Zoro has been steadily progressing in his proficiency with Enma throughout the Rooftop battle.
In Chapter 1001 he didn't use any haki at all in Rengoku Oni Giri:
After his attack, Zoro commented that he needed to unleash the sword a bit more:
He does make some progress. In Chapter 1002 he uses overflowing hardening with an Ittoryu technique:
In Chapter 1003 he finally uses hardening on all 3 swords, but only overflows it through Enma:
Maybe when next the fight resumes, Zoro will overflow his hardening through all three swords?
And as for damaging Kaido, Zoro already did that with Kokujo Ou Tatsumaki:
Zoro's Enma mastery has improved significantly during the Kaido fight. He doesn't need to fight King to master Enma, he's already doing that against Kaido.