I don't understand why you're linking this article. Do you mean that findings could be confused by serotonin production in the gut? If so, research on social anxiety disorder looks specifically at the brain:
"We found a significantly lower 5-HT1A BP in several limbic and paralimbic areas but not in the hippocampus (p = .234) of SAD patients."
Our discussion was too much serotonin (my stance) or too little serotonin (your stance) in the brain for people with mental health disorders (such as SAD).
I've come across findings that serotonin is mostly produced in the gut, about 95% of the body's production of serotonin is in the gut, and not a lot of serotonin is produced in the brain.
And so I referenced this article in an earlier post about serotonin being produced mostly in the gut:
I am implying that the hypothesis (which a lot of mental health professionals and doctors advocate) is that a chronically depressed or anxious person (diagnosed with a psychiatric disorder, such as SAD, through the DSM-IV) has a chemical imbalance in the brain - which normally implies the person's brain produces too much serotonin - actually, isn't very accurate, if the gut produces the majority of the body's serotonin, and not the brain.
I know you are stating that SAD (a diagnosed psychiatric disorder) is the result of too little serotonin, and an AD such as a SSRI would increase serotonin in the brain, however, that may be questionable if there are findings which imply depression (also another diagnosed psychiatric disorder) is the result of too much serotonin. Ironically when I look at your article, about too low serotonin, the source is also the one I referenced, which documented too high serotonin. **
However, I don't want to use this thread to make it you're wrong, and I'm right, about the serotonin theory. I will say that the chemical balance theory, in regards to serotonin, isn't well proven, as hyped by the media. If you have some extra time, this source does some extensive research to diss-pell the serotonin theory. The material is well written so it should be engaging to review, especially if you're prone to study psychological disorders such as SAD.
I will say that antidepressants work through the concept of neuro-genesis*, by creating new cells in the brain, which were killed off due to prior psychological stress. Without the antidepressant, a brain (damaged by prolonged psychological stress, which leads to problems in handling anxiety or depression) cannot produce these new cells on its own.
For me, this is a better theory on how antidepressants work, than to say antidepressants produce more serotonin in the brain, due to a chemical imbalance. If depressed subjects have too much serotonin, then too little serotonin may not be the best hypothesis as the the cause of mental illness, such as SAD. Ramping up the serotonin would then be unproductive.
Consequently the same antidepressants (such as an SSRI) can be treated for both anxiety (such as generalized anxiety disorder) or depression (such as major depressive disorder), so the antidepressant shouldn't be working on serotonin in the brain, instead the serotonin is being operated in the gut. Other parts of the brain, through neuro-genesis, are what the antidepressant is used for.