By the same logic, would they conclude that the vast majority of racists against black people are black because they are the ones who use the word "Nigger" (or derivatives) more? I have heard many instances of women calling each other slut as a friendly nickname, with no offense meant. Perhaps a bit misguided, but definitely not misogynistic on itself.
Though in all fairness, the study itself does not conclude that half of misogynistic tweets come from women. They report that half of the usage of the words 'slut' and 'whore' is by women, but they make a distinction between casual use and offensive use (and more distinctions within this). While not peer-reviewed, the study makes for interesting reading.