That article isn’t negative regarding serotonin nor SSRI. It is discussing a “one cause” (just serotonin) as reason for depression - which was perhaps a “cause” (I’m not in this field nor an expert). They do discuss relevant utd studies of the complexity of the receptors, other chemicals, digestion (inflammation) and neuroscience - the complexity of the situation.
Depressive states or thinking processes can be learned or “un-learned” OR can be influenced as well via the transmitters/receptors:
It in no way discourages the usefulness of the SSRI, but (imo as a layman) encourages not a professional laziness to treating depression or a reliance on, say family doctors, just prescribing long term anti-depression meds without therapies or (I would hope) diagnoses.