So there may be more (or fewer) female authors than that number, sure. The fact remains that this article completely disregards the publication rates for both genders---a factor which, if we use the 1999 numbers, shows a skewing toward female authors, and may very well indicate little to no skewing in one direction or another.

It's possible (and likely) that female authors have it rough, but this numbers presented here have too little context to show that. There are a lot of factors impacting which books get reviewed, including publication company pressure, book popularity, book availability, and reader interest. Drawing a line directly from reviewer gender to author gender seems ignorant at best and duplicitous at worst.