I can completely agree with this. It was a very disruptive (not the good kind) message - he likely exasperated the problems that quotas cause for other employees. The article mentions that the diversity manager should also be sacked and, while I never like to imply that should be done, someone did hand Edward a loaded gun; a person that was hired to prevent situations like that from happening.
It's an inherently unfair game. It doesn't matter how many crisis did she prevent, she didn't prevent one and that should not be enough to fire her.
Having said that, I assume this was not the first time he expressed his opinions about diversity hiring. The company should keep an eye on this kind of sentiment and use the information to guide its own behavior. If I knew diversity hires were creating such perceptions, I'd boost internal campaigns to better inform the workforce of the methods and intended results, as well as publish indicators that allow a fair assessment of its results.
I don't think, for a second, the bar was lowered. It's just that there is more than one bar.
The reason this person was fired was not because of the argument they outlined, but because they communicated this position via a long, widespread memo without appreciating the ramifications of doing so. The "hostile workplace environment" part isn't the argument per se, it's the fact that they're asserting this very publicly in a decontextualized way.
The employee could have written a 20 page memo about coffee being poisonous, and how it shouldn't be allowed at Google functions, with all sorts of scientific support they marshaled, and if they sent it out in the same way I guarantee that they would get checks on their "socially incompetent and unpleasant to work with" list, even if they wasn't outright fired for doing so.
Now, instead of coffee, you're talking about gender and sex, where there are people in the workplace who have reason to feel threatened. This doesn't mean those women are correct, or that the memo-writer's position is incorrect, but that writer distributing that memo in that way shows a lack of appreciation for social behavior.
Add to this that Google is a private company, and that employee is asking to be fired.
It's really simple: they could have kept the same opinions, and brought them up at specific times, with a limited subset of people, when it was practically relevant--such as when discussing a specific hire, or when the topic is being discussed with a limited number of need-to-know people.
Frankly, this is a time when I think how you talk about minorities in the workplace does matter. It's not thinking there shouldn't be quotas that's the problem, it's how you bring that up.