Yes, the point of my post was not to argue for the serotonin hypothesis of depression. If you think that, might I suggest that you missed my point, and you should perhaps reread with an eye towards absorbing novel information and viewpoints rather than trying to classify literally everything as either on a FOR or AGAINST side in a binary argument?
You were arguing that the absence of a blood test doesn't necessarily undermine chemical imbalance theories because of the complexity of the systems involved.