I disagree in the sense that I don’t think that “sin” is entirely dependent upon the concept of God. God is an idea. Sin is an idea. But for me, a “sin” is a crime against my conscience, against my moral code if you will. Littering is a “sin” to me, for example. But I think this is just an argument about definitions. If you define “sin” as a “crime against God”, then obviously you’re right. But it’s a tautology.
Probably netter to call it non-existent rather than supernatural. I don’t believe in the supernatural.