I will not say that enma has not made Zoro stronger but this isnt an accurate description.
nothing of that sort was ever said. if anything it was only specified that enma
draws the user's haki out more than what the user intends. handling it or
"taming it" &
regulating the amount of haki that's being put into each slash is a difficult process. there's a reason why no other swordsman could handle enma in the past, except kozuki oden ofcourse.
This is a false narrative spinned by taking a sentence out of context. Zoro said that
by the time he is used to the process of taming and regulating the amount of haki he puts into it, he'll be stronger. He has never once said that by using this blade he becomes stronger. He has been refining his haki control and the regulation process as he was training with it.
Luffy has gotten stronger while fighting and training with seastone cuffs in Act 2. So technically it made him stronger, so is seastone cuff a powerup too?
Kaido also openly acknowledged that
he felt Oden's Ryou when the scabbards pierced him. Oden never used any of their swords so how do you think that Oden's soul is living in their swords? Until the Oden's Ryou part is explained fully, please refrain from making random jumps in logic to justify your headcanon.
Moreover, this is the explanation of Ryou that Hyogoro gave to Luffy. A swordsman channel's
his own haki through his blade. So if anything , Zoro has been using
his own haki on his swords in the war.
The way I see it, Enma is like a training wheel until Zoro manages to do the same haki control + regulation thing with his other swords. Sorta like how Luffy gets stronger by training with seastone cuffs. Even if enma is channeling his haki outside, it still is Zoro's haki.