1) He shared it internally, he didn't go on a national talk show to broadcast his opinion to the world intentionally. Yes, it can get out, but that's the point - are you tryin to have a company that advocates only one point of view? And that's exactly his point.
2) He's not demanding change, he's not accusing anyone of anything...Someone should be allowed to share an opinion of mistakes a company is making, and ways to improve it, without being fired.
It's abundantly clear that most people didn't read it. But that's not important. The summary of his memo is an opinion that maybe the under-representation of women in tech is not sexism, rather it is nature taking its course. People can object to that without reading the whole memo. If they believe that it is only due to sexism, they have the right to believe that (and its probably the majority opinion).
True - I'm referring to quotes in one of the articles about a former senior engineer at Google saying that the Manifesto basically stated that women are not competent. He repeated again and again that individual women can excel, be as good or better than men in their field, and that generalizations have no bearing n any individual woman's ability.