I don't have the data since i didn't run the study, but i absolutely don't think it is hard to believe. Very very very few can outperform buying and holding over an extended period of time, while buying and holding is something everyone can do.
There have plenty of other studies that came to a similar conclusion, i remember once reading about a study done by a university done one over 66000 portfolios and they came to the conclusion, the higher the activity the worse the average performance was.
I don't have any studies or data at hand but i assume it shouldn't be too hard to find.