Enma's body isn't made of diamond, it is AS HARD AS the diamond. The Kusanagi was able to cut him, therefore Katsuyu's acid can melt him as well.
He was screaming in Pain, and the databook also states the Kusanagi was able to cut Enma.
Enma's body isnt naturally made of Diamond, but when he transforms he BECOMES a Adamantine Nyoi. Adamantine is a diamond substance, so he has all the properties of a diamond.
It was never stated that the Kusanagi could cut him. The original translation states that it would make him sore. It was stated in the databook that Enma in that form is indestructible, which means he cant be cut or broken.
The literal english translation when he transforms is Transformation: Vajra Nyoi. Nyoi means staff and Vajra, which is "Kongō" (金剛) in Japanese, is a mythical material that is said to be able to cut through any substance, but it itself, cannot be cut.
So no Enma wasnt and could not be cut by the kusanagi. It would only make him sore, and he still was able to disarm orchimaru nontheless.