correct, CFS has a heterogeneous and ill-defined etiology which may overlap with known affective disorders. Efforts to define a molecular mechanism have failed because there are no biomarkers. HPPD fits this bill. That doesn't mean these aren't real indications, but I don't have high hopes for a clear unifying mechanism for them.
More on challenges in CFS here:http://www.nature.com/scibx/journal/v4/n38/full/scibx.2011.1059.html