Aah, I think you're misunderstanding. If I understand
@Rivaille properly, we think that Enma would let Zoro improve his ryuo (proficiency, strength, control, etc.). We do not think that wielding Enma enables Zoro to do something he couldn't do before but that by using Enma, Zoro would be slowly strengthening himself.
Once Zoro has mastered Enma, if you replaced Enma with Shusui, we don't think Zoro would become any weaker. Once Zoro has mastered Enma, Enma becomes equivalent to just any O Wazamono sword. Substituting Enma for another sword of the same grade (which Zoro is acclimatised to) would not make him any weaker. This is why
@Rivaille and others don't believe Enma is a power up.
You think that Enma would enable Zoro to perform attacks he couldn't perform otherwise, they disagree. They think using Enma would gradually raise Zoro's abilities itself, but that holding Enma doesn't make him stronger.
I am personally not sold on Enma as G4. I think it's plausible and wouldn't be surprised if it happens, but it's not something I'm convinced of yet. I'm mostly taking a wait and see approach. The true nature of Enma would become apparent later this year.