Why E?

^^^^

For questions like this that reference a specific line, I would usually check the immediate context. We are told that the adventures of the Piccaro begin when he yields to his own roguish impulses and indulges in vices that outrage society- although these vices are secretly common. The piccaro is being true to himself in that he does not hide his vices, but does them openly- and in doing so holds a mirror to a hypocritical society. The term authenticity does not refer to the fact that he does the same thing as everyone else, but that he does not try to hide it- that is he follows his instincts in the open with no hint of self restraint or shame. In this way, he is true to himself in a way that his peers are not.