It wasn't that it was sexist, it was that it stereotypically characterized a gender in an effort to get Google to stop being so proactive in their compliance with EEOE laws.
I don't think it said anything about innate superiority - as I recall, it was mostly about innate interests and bell curves, and posited that the reason there were less women in tech than men was because there were less women interested in things than there were men interested in things (with the idea that more women are interested in people rather than things)
Not really the point either. If at the extreme end of the spectrum for more interest in things rather than people you have a significantly larger population of men than women, that's not really a sexist thing to say.
It's logical that people extremely interested in things over people become engineers, so with everything equal you'd have a lot more male than female engineers.
To balance that, you'd have to make other benefits great enough through incentives that people become engineers even if they are more interested in people, rather than things.
Lacking such incentives and having a large imbalance of male vs female engineers, it'd also make sense that there would be a large imbalance of male to female engineers at Google, entirely without the presence of gender discrimination.
None of that was what I would call sexist. However, Google is in compliance with EEOE laws when it attempts to encourage more women to join the company in all capacities in large part because of this imbalance, and even though Damore is arguing (and even if he's right) that this imbalance is not caused by a lack of equal opportunity, it doesn't matter.
Maybe not exactly, but the above quote pretty clearly outlines that the NLRB agrees that the memo was supporting a sexist viewpoint.