> 88.2% Google-referred traffic.

I don't think that's a "pure" statistic. Perhaps StackOverflow users rely on Google returning StackOverflow answers? I know I do; for example, I often append stackoverflow to my programming queries because I know Google will give me StackOverflow answers. Although the referral is Google, I am forcibly limiting results to include only StackOverflow.

even if they would clean it up (add every query with a brand term to a separate segment) i would be very very very surprised if it would be below 70%

SO is designed so that google is their home page. Joel Spolsky talked about that, year or two ago, in one of episodes of SO podcast.