It is not irrelevant. It seems you and many others have missed said google manifesto's writer's point; namely, that striving for 50-50 men/women in STEM might be unattainable in a society predicated upon individual choice and equality. IF we allow men and women to choose what they wish to study and what career they wish to pursue, then we must accept that the outcomes in any given field might not be a perfect 50-50 split. THIS was the point of the manifesto, that there are inherent fundamental biological differences in individual humans based on brain chemistry and that this manifests itself as statistically relevant trends between men and women.
 
WE are not all alike. And forcing us to be is the opposite of choice. Many biological differences might be minute, which is why no one should be restricted from a job or forced to do a job based on their gender.  Mandating 50-50 gender ratios is exactly this: forcing people.