It is a study about making associations, not identifying causes. Its implying serotonin chemistry might be involved, but nothing is proved. It does say this -
Anti-5-HT antibody positivity was significantly associated with increased scores on hyperalgesia, fatigue, neurocognitive and autonomic symptoms, sadness and a flu-like malaise.
This is an association. It might be causal, it might be partly causal, it might be coincidence, it might be some other factor the study did not look at which is causal, and associated with the autoantibodies and symptoms.
All it really says is this is a line of research that might be worth pursuing.
I wonder if its coincidence that this kind of shows the one-third/two-third split I have been seeing since the Lights' exercise research was first published.