This conversation does not occur in a vacuum. For most of the history of writing in English, a casual everyday sexism has been baked in. The presence of that sexism has done harm even if few authors have really cared about it. We're lucky, that now we can't really pretend this isn't a thing. So the reasonable assumption to make is not that a text was unintentionally sexist, but rather that the author didn't care enough not to be sexist. It's really easy to write "humanity" rather than "man".