1. We know that the symptoms of many different varieties of mental illness have been helped by the administration of SSRIs and other drugs that alter brain chemistry.

2. But what we most assuredly do not know is that mental illness patients have detectably different brain chemistry from baseline.

We assume from (1) that (2) follows, but we have no actual evidence of (2). Herein lies the problem. We cannot make a scientific statement on (2) predicated only on (1).

In this specific sense, OP is correct. And the downvotes are unfair. And more research is needed.