He's not "Holding Enma back". He's holding back the amount of Haki Enma is exuding from him, so the Sword doesn't bleed him dry.
Enma exudes a persons Haki on it's own.
This is showcased by the fact that it almost turned Zoro's Sword into a husk by pulling out a massive ton of Zoro's haki.
In order for for Zoro to not become a husk, he has to hold back the haki Enma exudes.
Even holding back that Haki, Zoro is still exuding a massive amount of Haki with Enma, in every attack. That is boosting his AP tremendously.
Like I said before, it might be harder for him to fight, considering it is splitting his focus between combat and keeping Enma from bleeding him dry, but even if he is being "Nerfed" in one way, he is being heavily boosted in another way. Hence, it's a double edge sword.
If you take Enma away from Zoro, and allow him to be able to solely focus on combat, he can't beat King, because he needed that boost from Enma to reach the next level. So it doesn't matter how his combat is being "nerfed" because he needed to come to an understanding about Enma in order to win the fight.