If I say something like that, I will (rightly) become a social pariah immediately and nobody would want to be around me.
Same thing with that guy. Yeah, you can joke about Trump, and you can joke about Hillary's emails, but saying that women are inferior due to biology makes you an unpleasant human being to be around and I don't want to be in the same room as you, much less on the same team or the same company as you.
If they hadn't fired his dumb ass, it would have sent a very bad message to all of Google's women employees.
There was more than just "joking." There was a hostile atmosphere for a lot of my peers who voted for Trump, akin to how women might have felt after this latest memo. The thing was, that it was perfectly "acceptable" (within the larger "Google culture") to create an environment hostile towards conservatives, with no Management backlash (probably because a lot of them felt the same way). The point was they didn't find that kind of "harassment" towards conservative employees problematic at all.
It's okay to express an opinion that is targeting a group of people (conservatives/Republicans/Trump supporters/etc.) if you belong to the right ideological group (progressives).
Don't think for an instance that this is the first case of controversial content posted internally. It happened all the time when I worked there, just that the group getting targeted wasn't in the majority in the social dominance and power hierarchy.