If that's true, then an AdSense publisher should never conduct any non-AdSense advertising on the site. We have no way of knowing which advertisers' ads are being displayed to the various visitors. In other words, any advertiser could be advertising with us via AdSense and we wouldn't know it.
It's very easy for some of the publishers and advertisers to find out about each other just by publishers viewing which ads they see when they visit their own site.
But I don't think that's a big worry for Google. The technology behind the PPC concept and the contextual matching eliminates the majority of publishers from offering advertisers the same experience.