I also suspect there may have been some flaws with the study, but I am a big believer in reading original source material. So, I am going to hold off until I have read the entire study before I speculate.
I will say this, by way of example only: I loved Chris Cleave's LITTLE BEE (literary fiction, female main character) and enjoy Jamie Freveletti's thrillers featuring Emma Calridge (female). But, I also am a huge fan of Lee Child (protagonist is Jack Reacher, male) and Michael Connelly (particularly the Mickey Haller books, male).
In fact, I would venture to guess my bookshelf is about evenly split between books with male and female protagonists.
What do you think?