This seems to me as an important discussion to be had in the modern workplace, and not one that deserves a person with an opinion, either for or against, to be fired over.
People are pointing to the fact that some workplaces those with less than 50% of their employees as women are somehow lacking actual representation of society is misleading at best. A workplace where a specific skill set is required to perform your duties effectively, should in actuality reflect the population of people with the degrees associated with that skill set. In 2013 only 18% of computer science degrees were held by women, and by that metric google's 30% women employee value seems fair. Granted degrees may not be necessarily needed to function well as a programmer and that there are several other positions available at an organization as big as google. Nevertheless a qualifying metric that helps shed light on why it is not 50% women.
Beyond that, there is a problem with creating an echo chamber, one in which almost totalitarian type leadership of organization will seem absolutely normal as what we see in this specific instance.
Strength in diversity comes from acknowledging the strengths and weaknesses of groups that make up the diverse population, all this memo did was bring this very idea up for discussion. Nothing wrong with that.
When I heard about this issue people claimed that he said "women were less fit for this job" and that his comments were "promoting violence". After reading the memo, it was clear some people took it way out of proportion.
Before he started he clarified that sexism exists, and that it is an issue that needed to be fixed. HOWEVER, he thought that the way Google was handling it was discriminatory in itself. One of the things he talked about was that Google was lowering standards for minorities and women, which is incredibly problematic. I personally think that it is ridiculous to lower standards for minorities because it implies that Google thinks they won't perform as well as a straight white man. If anything, Google should check itself before accusing this man of being discriminatory.
He also made the (true) statement that Google has a bias against more Conservative ways of thinking. As he said, neither side is 100% right, and although we don't like it, we need both sides to have a functioning society.
Throughout his memo he made clarifying statements such as "this is not the case for all women", "this is just from ___ perspective", etc. If he was truly sexist or promoted sexism, he wouldn't be implying that things differ on a case-by-case basis.
You could argue that his statements on the tendencies of women were sexist, but I doubt that was his intention. It is true that women are less likely to negociate their wages, and that they prefer certain fields of work. I doubt it is purely biological differences that cause it, but it wasn't as though he said women shouldn't be in these fields. He wanted to discuss ways to make these jobs more appealing to women.
So overall, no, his comments were not sexist (or discriminatory).