I am very skeptical of his claims in this article. The social determinants of health are undeniably important especially in the field of public health. There is some evidence that income inequality is linked to depression rates such as here for women and here in aggregate. Those of low socioeconomic status are more likely to have depressive episodes so the evidence is out there.

That being said, it is pretty obvious that psychology is not his field. There appears to be a lot of conflation of unhappiness and depression, which is not good. Clinical depression is not simply being sad in relation to bad things. Social determinants such as low-income and income inequality affect the prevalence of Clinical Depression but they are not the sole cause. Also his discussion of therapy isn't really consistent with standard practice. He largely ignores medications which do work very well, in fact according to the APA's guidelines "An antidepressant medication is recommended as an initial treatment choice for patients with mild to moderate major depressive disorder [I] and definitely should be provided for those with severe major depressive disorder unless ECT is planned [I]."

His broader point about unhappiness is not exactly new, one could say that it is old as capitalism itself. You can certainly find strains of this thought in Montesquieu. His points on mental illness are too strongly stated for the evidence existing and belie a lack of clinical knowledge.