I read the memo and it didn't sound simply anti-diversity but anti-forced diversity.  I agree that people are naturally drawn to certain jobs, tasks, etc and that and that can lead to a lack of diversity.  However, the memo didn't say minorities in a role are less suitable or less better for a job.  It seems to question how much value a company can derive from throwing money and resources at forcing greater diversity.   People with different backgrounds can provide different perspectives on things but the difficulty is that the value is often times subjective and difficult to equate in terms of real dollars.  
While I think the google employee's opinion should be respected, corporations are not free speech zones.  It's just reality that potentially controversial opinions and ideas need to be run up the right channels within the company because as illustrated here, certain topics can blow up and impact the company negatively.