There's a bit if immaturity in the manifesto despite it being very rational and well-reason. In my mind, it's the immaturity of a rich kid who is very intelligent, having had the best teachers, and also well meaning and trying to do good.
To understand what affirmative action actually tries to do it must be understood that there are 2 primary types of moats. One is a "Test Moat" that tests required abilities. A firefighter being able to carry 100 pounds is an example. The other is a "Privilege Moat". This is primarily based around gating access to resources to a privileged few to create abundance. This gating is socially created and maintained.
These moats have a lot of overlap. Asking a candidate if they know what a 'function' is is a simple type of test moat. Asking a web developer to create a red-black tree on the fly is a privilege moat.
(Google is in a bit of a strange spot as there is a widespread belief that web developer who DOES know how to create a red-black tree on the fly is better than one who doesn't. Most companies do not have the attraction or excess capacity to actually care about this, they take what they can get. (or stupidly mimic the Google process) They don't have the bandwidth to care about privilege moats.)
The moat being sacred is the capstone for the delusion that allows selfishness to be hidden and prevents growth of moral guilt. Privilege moats not only gate access but also must assuage guilt.
Test moats naturally grow into privilege moats over time. Especially as the tested skill becomes commoditized. The existing base naturally feels threatened.
Affirmative action is about de-sacredizing privilege moats. It is primarily destructive in nature. There's no nice way to shit over something. It's primary purpose is to destroy specific types of culture. Even today, the culture of "White man's Burden" is still being destroyed and is generally considered a good thing that it is.
In Google's case, de-sacredization may primarily be about recognizing that certain skills are nice to have but not NECESSARY. It lowers the bar because new candidates will no longer be able to keep up with passionate Haskell discussion during lunchtime on break from adjusting CSS.
Going a step further, it is de-sacredization of the idea that your work must be your life. That you must live and breath code to be a "real" coder. This idea CANNOT co-exist with the idea that coding needs to become a core competency. I don't live and breath reading and writing but I still read and write.
This touches on the author's valid point that certain positions have more stress and that the development of male gender roles to be more feminine has lagged behind. (I have a pet theory that this is driven by lack of economic incentive and is due to Americas obsession over money. Cultures that value psychological and familial happiness/health may be further along in developing the flexibility of male gender roles)
However, this valid point is mixed up with the real privilege moats that should be adjusted. The author is overall well meaning and representative of valid thoughts and ideas and deserves none of the flak he has taken. He simply had more courage. Bring up these ideas and absorbing them to create a counter-idea is what can inspire real change. Many self-described liberals are very hypocritical in this respect, creating privilege moats based on capacity for self censorship.
What I've mostly described here are bars that are lowered because they are needlessly high. However another motivation for lowering bars are their second order effects. It creates role models and opens up a potential life path in the eyes of children. It makes a domain seem friendlier and more enforced (less unsaid social requirements).
It can also be somewhat apologetic in extreme cases. The affirmative action movements for African Americans in the U.S. and the lower castes in India come to mind. Without affirmative action, the initial momentum and privilege for certain subgroups would continuously compound and maintain tiers of ability/people. All tiers would theoretically grow together, but the relative distance would grow. Capitalism does this too via the initial capital allocation.
Growing relative distance creates a sense of unfairness and unless there is infinite abundance, market auction dynamics will cause the upper tiers to have almost everything. Very poor people can have instant connection to their friends via facebook but still be fighting a headwind when trying to pull themselves out of poverty. (There is expectation that the government maintains a decent privilege moat so that its citizens can live abundant lives, enforced with guns if necessary. The only governments that can't do this are those where moats of ineffective because the issue is already within the moat. China/India have their hands full.)
Notice how affirmative action for things like education (candles lighting candles) is much more accepted than affirmative action for capital (zero sum). The tech-gender-gap story is somewhere in the middle. Does tech create enough abundance that it can effectively be infinite for all people int he bay area? Is it artificial scarcity due to greedy execs? Will lowering the cultural bar for Google cause it to become uncool and cause the cool smart people to leave?
who knows...