Even if Enma is a power up, so what? I don't really see what's the problem of those saying that Enma is not a power up.
We saw already when Shusui appeared that Zoro made a bigger slash even tho he didn't mean to do it. Now Enma has it's own speciality, it sucks the owner's haki and releases it, and there is probably something more since Kaidou sensed Oden's presence.
Now, Is Enma a power up? Yes, since Zoro hasn't shown us that he's able to do that with any of his other swords, maybe there is a limit of how much they can release but Enma doesn't have that.
So, once again, it's not a problem if Enma is a power up, Sanji has the raid suit, Luffy has his gears, Zoro has his swords, Nami has her clima-tact, etc.
I really don't see why they try to say that it's not because of Enma, and why is that even a problem?
And btw about Oden's presence in Enma I believe that it has to do with Oden's haki tbh, we knew that when a sword goes through countless battles it becomes a black sword permanently just as Shusui. Now swordsmans can transform swords to black swords through Haki(just not permanently), so i believe that through many battles with Enma, Oden didn't reach to make it a black sword but some of his haki remained there