This study made a big splash in the news.
Here's the inside scoop: Few people in the field ever believed that depression was caused by a "seratonin imbalance." Far too simplistic. There is no single "depression" anyway.
What is termed "depression" is a grab bag of miserable signs and symptoms which can be associated with some (undiagnosed) cancers and Bipolar Disorder to neurotic and/or personality vulnerabilities.
SSRI meds help some people with depression remarkably well, but probably not most.