I think you are misrepresenting what she says in this excerpt. She describes what online harassment may entail and repeatedly calling someone a liar or saying someone sucks can be ways harassment materialises. Moreover, this can even be true if the thing being referenced is actually a falsehood the target of the harassment has said. So, for instance, in my first posting online, I enthusiastically said there must be a Yoshi game for GameCube coming, because there was a Yoshi-themed icon on the GameCube manual. As we all know, I was wrong. Moreover, it was pretty stupid to conclude from an examplary picture on the system's manual that a new Yoshi game must be coming. However, if people were always responding to me, on unrelated issues, calling me a liar because of that one thing I got wrong in 2002, this would consitute harassment, even though it is true that I over interpreted that image, greatly.
Sarkeesian probably is in a situation where mistakes she made in previous works are again and again being used to paint her as a purposeful liar, as an insincere person and as someone providing a distorted view. This is a form of harassment, especially if the same person is bringing up the same mistakes - honest ones, deliberate ones, or ones that are not actually mistakes, but just perceived as such by the attacker - repeatedly to talk her down. Indeed what detective GAF did (and their successors on Resetera) is certainly harassment, to collect histories of posts one may not like, often misrepresenting them even upon quoting them, by being very selective and thus character assassinating people instead of arguing faithfully on the current point of discussion.
Of course, it is difficult to deal with this in a fair and balanced way, and it should not be a process of jurisdication (except in severe cases), but of moderation. Something Twitter is notoriously lacking in, which, combined with the short message nature and the social accumulative functionality makes Twitter a particular minefield.