Intellectually dishonest scholarship.
While it may well be that federal employees are over compensated compared to private sector (and I suspect they may be, since private industry has chopped health and retirement benefits), you can't prove it from this analysis. Compare like jobs - accountants to accountants, lawyers to lawyers, secretaries (does anyone still have those?) to secretaries, doctors to doctors, loan officers to loan officers, etc.
I do think we have enough evidence to conclude that the author of this study is overpaid compared to government workers.
One piece of the excess is the myriad of huge staffs working on amazingly insignificant projects in the government. We do not need separate staffs in every agency of every department coming up with EEO training, etc. All that common training could come from OPM.