They weren't actually saying what caused the bio-chemical imbalances, though some of it could be caused physical problems with the brain or surrounding hardware. Take the drop in serotonin levels. Serotonin is produced outside of the brain and has to pass through a membrane in order to reach the brain and be used.
It seems that they are trying to attack this problem from all the possible angles they can, and this is just one small experiment with two dozen people using a technique that is fairly new. They weren't trying to prove much beyond showing that they could indeed trace the levels of certain chemicals in certain parts of the brain, and that they could find a difference between the two groups of 12 people.
You could very well be right that the problem could be totally psychological and that the chemical changes are self induced. They aren't discounting that when they prescribe serotonin uptake receptor inhibiters or whatever they give out to change the balance of chemicals, just that they can mitigate the symptoms with these drugs. They are looking at all angles and undoubtedly psychologists somewhere are working on the exact same problem from a different angle.