Yes. He assumed that she was incapable of writing the book, in the first place, because of her biological sex per se.

It was mansplaining-on-steroids.

That interaction was hyper-extreme. Solnit was sharing with all of us one of the most egregious examples she'd endured. I don't think she was trying to excuse or minimize the comparatively subtler examples of mansplaining that can and do occur.