It's not new to anyone who has studied psychology that the "low serotonin" theory (monoamine hypothesis) of depression is wrong, but saying "not caused by chemical imbalance" is not a great way of phrasing that. Hormones and neurotransmitters play a role
Conversation
Why do anti-depressants seem to work for some people? Nobody really knows, but there are some promising theories. E.g.
1