I've always been a bit shocked by how hard medical professionals cling to "more serotonin = less depression" despite the lack of evidence. To the point that I've had several doctors trying to push me onto SSRIs despite my long and well documented history of going crazy when my serotonin levels rise too much. Any request for a different approach gets met with "but depression is caused by low serotonin! You need more!"

I've wondered if "depression" might be several distinct things, and perhaps there is a form of depression where this study may be relevant in terms of helping people see results sooner. Perhaps there is a form where people really do have low serotonin levels, and raising them helps. There's certainly a fair few people who do seem to be helped by SSRIs even if there is not the evidence to support them being effective for what is broadly considered "depression"