It is only an illusion. The panels made that very clear. Kaku saw the illusion and then we saw Zoro normal. Then when we saw Zoro in that form he calls it an illusion again.
An illusion doesnt create stronger cuts, thus it cannot possibly be an illusion.
It is only Kaku's guess because he didnt understand it.
All evolutions of humanity are real instead of illusions.
It would not triple Zoro's power if it were just an illusion.
The power-up is real and tangible.
Actually I'd say we don't know it. We never heard about Oden had any issues with Enma. It seems he uses it since a very young age without any trouble. Probably he didn't even had haki at time.
I still think it has something to do with how the owners pass the swords forward. Zombie Ryuuma specifically passed his sword to Zoro and he said something to the sword. So Zoro had an easier time taming Shusui. Oden passed his swords to his kids. So when Zoro tooks it Enma is rejecting Zoro since the will of the previous owner was to give it to bitch Hiyori and not Zoro.
I still think it has something to do with how the owners pass the swords forward. Zombie Ryuuma specifically passed his sword to Zoro and he said something to the sword. So Zoro had an easier time taming Shusui. Oden passed his swords to his kids. So when Zoro tooks it Enma is rejecting Zoro since the will of the previous owner was to give it to bitch Hiyori and not Zoro.
It means it can see your desire and ambition.
And then it scales to that ambition and charges you accordingly.
The fact that Oden used it normally since baby years proves that his ambition wasnt that great.
And that's why Enma that can see into him didnt demand from him as much as it demanded from Zoro.
It will obviously charge a lot more the one who desires to be the strongest compared to the one who left Wano for information about past.
Shusui is satisfied with what Ryuma achieved, it is a sword that "lived" its life and served its purpose. it has been put to rest.
Enma was thirsty for Blood since Oden's failure against Kaido. It will not rest until Zoro forges it black.