Mind altering drugs allows one to glimpse what kind of person one could be if they where e.g more empathetic or less anxious. It can help people better understand such feelings.

I can understand the “true self” narrative, when one loses their constant anxiety for on a trip for example, those feeling suddenly start to feel different in sober life, as if they are no longer a part of you but instead are with you. Hence why you discover your “true self”, who does not have such negative feelings.