First of all, there are definitely Cones of Non-Communication between the AdSense people and the Search people. Always have been. Matt (and other Google types) have admitted as much, and we've seen evidence of it. There are valid arguments for and against that, but it doesn't matter, because that's the policy and the culture, and we need to get over it.
Second of all, Google has always shown that they're willing to take a short term hit (on things like AdSense and AdWords) to get to a longterm gain. Always always always. Whether or not they actually get there, that may well be what they are going for. So yes, I believe them when they say it's possible to go overboard with AdSense to the point where it affects rankings.
The main thing is, stop trying to micro-analyze everything Google says, because Google deliberately says things that can never be pinned down to a single cohesive thought or action.
Instead you have to rely on your common sense. Every single one of us have seen sites that are so bloated with ads you have to hunt to find any content. Don't do it like that. If you have to take anything away from Google, take the "long term" attitude - it's better to give up a few pennies (or dollars) of ad revenue in order to keep a reasonable amount of your content above the fold and keep your users happy. Google will do what Google will do. You have no control over that; all you have control over is what YOU do. So just use your head.
And remember that it's *still* only one of a whole bunch of factors. Sites without any ads were pandalyzed, and some sites loaded with ads survived.