What a weird study.
The study participants had to be 15 years of age or older and present with abdominal symptoms of abdominal pain, diarrhea, or constipation that fulfilled the ROME 3 criteria for IBS. The inclusion required an absence of red flag symptoms (namely, anemia, weight loss, gastrointestinal bleeding such as hematemesis, melena or hematochezia, or family history of colorectal cancer). They had to present with complaints of headache or migraine, joint or muscular pain, or anxiety and depression.
The inclusion criteria were approximately:
1. abdominal symptoms that equalled IBS, with nothing suggesting another bowel-related diagnosis,
and
2. at least one of: headache, migraine, joint or muscular pain, anxiety or depression (they termed these 'psychosomatic illness')
They then assessed the prevalence of each of the psychosomatic illnesses.
and then, if I have understood right, they concluded that psychosomatic illness is very common in patients with functional gastrointestinal disorders.
From the findings of this study, we can observe that there is a close relationship between functional gastrointestinal disorders and psychosomatic symptoms in our local patients.

Sometimes I am surprised anew at the stupidity. Unless I have got it wrong and it is me being stupid.
We have no idea how many IBS patients they had who had no 'psychosomatic symptoms', because they only selected patients who had psychosomatic symptoms for the study. So they can't conclude anything about how common psychosomatic illnesses are in their IBS patients. And 100% of their sample had one or more psychosomatic illness.
And that's even before we start considering whether a headache or a migraine or joint pain or the others are necessarily psychosomatic, or if the instruments they used to decide if someone had any of those conditions were accurate. Looking at the headache definitions for example, a person may have qualified as having headaches if they have had one. And presumably having IBS might mean that you exhibit a few anxious or depressed symptoms without necessarily having anxiety disorder or clinical depression.
Some nights this game feels like shooting fish in barrels.