That's indeed a nerf. Losing Ryuma's haki, of course not at its full potency but still Ryuma's haki.
I feel like it could be a foreshading of Zoro's upcoming black blade twist. That would be a treat if true.
Of course I don't expect it to be impending because not even Roger had supposedly attained so.
Indeed, Zoro removed the Sword of Legendary Ryuma that had his Haki
I believe it's a Nerf however Enma's Ability is that it forces the Haki of the user out so it helps the User brings out his full potential & Haki
In other words, Zoro got a Nerf actually, not a Buff because he needs to make his own Black Blade & not keep an Already Black one
Also Enma is same Grade as Shusui but if turned Black it can become one of Supreme Grade Swords
So basically Zoro sacrificed the Greater Sword in exchange for Greater Potential
It's like this:
Zoro with Black Enma > Zoro with Black Shusui > Zoro with Non-Black Enma
So Zoro got Nerfed so that he receives a Huge Buff that makes him better than what he initially was later on when he makes Enma Black