I know we'll have to wait for the official translation for me to discuss this point any further, but Zoro did say the moment he gets used to it it will make him stronger; it didn't imply Enma having to be completely mastered and become a black blade by his hands in order for that to happen
As I said a different gimmick.
Shushui was heavy and hard
Sandai wanted to cut everything
Enma draws the user Haki to high quantities without limit.
Enma forces Zoro refine another aspect of swordmanship as the other swords did in the past.
See? Is like each sword was a training.
He will grow obviously but it's not cuz the sword it's OP .