I concur with the authors that this study needs to be viewed with extreme caution.
There is no solid conclusion to draw from this particular study.
I'm open to a variety of possible results from releasing free books, but IMO this study is just too narrow and targeted to produce viable results. It also doesn't look for a "halo effect," for lack of a better term; e.g. in many cases, the goal behind releasing a free ebook by an author is to increase sales of other books (paper and ebook) by that same author. This study doesn't look for that effect at all.