In the case of sexism there is a lot of evidence to suggest that it runs through society itself. To be clear Anita doesn't blame individuals most of the time, she merely points out that a lot of it is the result of tropes and cultural baggage. To pick a random example female characters often have stereotypical female attributes, such a lipstick, eyelashes, pink clothes, a bow in their hair and high heels, because it's a quick and easy way to inform the audience. It also works when you have few pixels to work with, which was the case with early games.
What she is arguing for is for designers to be a bit less lazy and get away from these tropes. Sometimes they do and it works well, and she gives credit where it is due. She does have a point though, there are things in our society and shared culture that are sexist, and by being aware of them we can at least try to change them.