The Adsense team have clarified this point themselves in a comment to the original article:
"The 68% revenue share for AdSense for content applies to all online publishers, and is not an average revenue share. If you're showing AdSense for content ads on your pages, you're receiving 68% of the amount advertisers pay for those ads. While the revenue share can vary for some major online publishers with whom we negotiate individual contracts, these amounts are not in any way averaged together. Also, there isn’t anything additional taken off the top. You get 68 percent, period." [http://adsense.blogspot.com/2010/05/adsense-revenue-share.ht...]
"The 68% revenue share for AdSense for content applies to all online publishers, and is not an average revenue share. If you're showing AdSense for content ads on your pages, you're receiving 68% of the amount advertisers pay for those ads. While the revenue share can vary for some major online publishers with whom we negotiate individual contracts, these amounts are not in any way averaged together. Also, there isn’t anything additional taken off the top. You get 68 percent, period." [http://adsense.blogspot.com/2010/05/adsense-revenue-share.ht...]
My original skepticism was unfounded.