They are not “injecting a molecule into someone.” They were assaying the brain tissues of dead people who had been depressed and comparing them with the tissues of dead people who were not depressed.
They also did clinical trials, giving antidepressant medication to depressed patients and comparing the levels before and after treatment. When the patients had an improved mood and functioning, the researchers did correlate this with an elevated level of this molecule, which had probably been raised by the introduction of the medication.
“They” do not necessarily think “we are nothing but a bunch of chemicals and electrical impulses,” as you say, but they do acknowledge that some psychiatric problems may be the result of biochemical malfunctioning. You will grant that the biochemistry of your stomach or liver or kidneys can malfunction, causing disease, so why is it hard to believe that the most sensitive and complex organ in your body, the brain, can have its biochemistry go awry?
Your writing is like a cool refreshing stream in the woods.
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.