On the other hand, the author might have referred to the fact that shame won’t let people reveal themselves to others and once they are given the chance to hide in the dark, or behind a mask in this context, they won’t mind confessing the truth to the audience, since there won’t be any consequences. It is like when people confess to the priest, they stay in the dark of the chamber and cannot see the priest’s face, so that they can gather their courage to talk about their sins.

Whatever angle you look at this issue from, you cannot deny that the author has a good grip on psychology. I am more inclined towards the second concept that I provided. However, in psychology what is referred to mask is the behavior you choose to acquire to survive in certain situations or deal with certain people. It’s not your actual self and it’s usually to conceal The Self. (check Persona/Archetypes)