I believe what the authors are saying is that IBS is a disorder that has physical and emotional discomfort increasing such things as stress and somatization with a psychological basis that perhaps can result in many non-bowel related symptoms. Techniques such as hypnosis and CBT which have been shown to be quite effective for managing IBS symptoms and they also seem to be effective in managing non-bowel functional GI symptoms too.
I don't believe that anywhere in the article did they suggest that functional GI disorders themselves are psychological. The authors only suggest that the non-bowel symptoms may have a psychological as well as physical components. Most of the research that comes from UNC definitely doesn't show these researchers believe functional GI disorders are psychological disorders.
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS