Top Reddit comment summarizes this finding:

Low serotonin levels are frequently linked to anxiety. Most of the popular antidepressants and anxiety drugs are Selective Serotonin Reuptake Inhibitors (SSRIs), meaning they stop the breakdown of serotonin in the synapse, increasing the activation of serotonin. This is great for someone whose anxiety is potentially caused by low serotonin levels so they're typically the first drug that someone is prescribed. But this study shows that people experiencing anxiety may have HIGH serotonin levels, so giving them an SSRI would only make the issue worse. 

There are atypical (non-SSRI) anxiety and antidepressant drugs, the most popular being Wellbutrin. So there are options for people who don't respond to SSRIs.

400+ Reddit comments:

https://reddit.com/r/science/comments/504n87/understanding_the_unhappy_side_of_serotonin/

Ray Peat has been saying that serotonin is really a stress hormone for decades.  It makes you feel better in the short term and worse in the long term.  In other words the mainstream science view is completely wrong.  For anyone who has observed the fact that almost every crazy mass shooter is on an SSRI, this isn't totally shocking.  There is a ton of evidence for Peat's view which I'm not going to summarize here, except to say that some of the latest drugs approved for extreme trauma (read: stress) in the ER are serotonin blockers.  These drugs are very effective, and if you learn a bit about the biology you begin to see how this would not be possible if the mainstream psych view of serotonin were correct.  There are credible rumors that the big drug companies have actively suppressed research backing this up.  One of them recently ejected a board member for speaking about it, if memory serves... wish I could find that link.

I can see how serotonin would be stressful if the dosage were too high.

I guess how much serotonin a person can handle varies. 

"Serotonin research is relatively new, but it rivals estrogen research for the level of incompetence and apparent fraudulent intent that can be found in professional publications."

http://raypeat.com/articles/articles/serotonin-depression-aggression.shtml

Kind of like most of the weight loss research currently being done.