Your point is reasonable but misses the underlying point being made by “I Want the USA back,” namely that a treatment can be effective but that doesn’t mean the treatment actually addressed the underlying issue (e.g., some bandages help “treat” wounds but knowing this tells you nothing about what initially caused the wound). In this case, being deficient in a molecule and then replacing, via external intervention, that molecule tells you nothing about what caused the initial deficiency in the molecule.
Clinical depression is different from being sad about things. The underlying cause of clinical depression can be an imbalance in your blood chemistry.
There are a lot of different factors that have the potential to cause psychiatric disorders: life circumstances and tragedy, diet, lack of sunshine and Vitamin D, genetic predispositions, trauma, spiritual problems—the list goes on. As a Catholic Christian I certainly see evil forces as contributors. But that doesn’t mean we shouldn’t use every weapon in the gun cabinet to fight it, and along with spiritual weapons we should use medical research as well. For even if there is a situational contributing factor, sometimes we can’t even reach emotionally ill people verbally to help them with their problems. Sometimes medications can lift them up to the point that they can accept help. Drugs do have their uses.