No, there's something wrong with your reading comprehension.
>things he mentions have a large enough effect to measurably affect the gender distribution among Google's employees,
That's not one of the points of the memo. One of (the primary) point is to potentially explain a gap instead of relying solely on the assumption that the sole cause is discrimination, or "bro culture" or whatever trash rags like Model View Culture are calling it.
The memo is not about the ability, nor the preferences, of Google employees, specifically. It's about the ability, and/or preferences, of a potential hiring pool. And, if you actually read it, you'd know that the author used a qualifier: "may." As in:
"Women are more likely to have these personality types and these interests, and this may explain the gap." As opposed to "all women have these personality types, abilities, and interests."
If women are coming out of university with a related degree or solid references for a software engineering career, at rates approximate to their original distribution (half of the population), we could say that there's obviously some sort of discrimination going on. But they are not. If they were, I would be prominent among the many who are outraged about the distribution of women in these particular careers. I believe the author of the memo would, as well.
But, again, they are not. And it is unhealthy to blame it solely on sexism, if at all.
I'm not surprised at the outrage, really. You get that when telling faux-progressives, who worship at the alter of tabula rasa social science models, that the mind is not a blank slate.
You see it in the Scandinavian countries, where certain "gendered" career choices are more clear than ever. But if we listened to the outraged mob, you'd think that these countries were some patriarchal backwater. But that's obviously not the case.
Another thing you'd have to ask yourself, if you were being honest, is: why "engineering"? The gap is bordering on nonexistent, if at all, in fields like law or medicine. So why is that this deficit exists in these vaguely related (if only by name) "engineering" careers. That's another thing the author decided to ask, and was shit on for it.