One misconception that we’ve seen in the last few weeks is the idea that Google doesn’t take as strong action on spammy content in our index if those sites are serving Google ads.
That's not quite what I've been reading. I believe the more common claim is that Google has a disincentive to algorithmically weed out the kind of drivel that exists for no other reason than to make its publisher money via AdSense. It's about aggregate effects, not failure to clamp down on individual sites. Or, put another way, it's not if certain sites are serving Google ads, it's because that kind of content is usually associated with AdSense.
AdSense is definitely a problem for search quality. It creates the same imperative for the content farm as Google Search has: get the user to click off the page as soon as possible. And the easiest way to do that is to create high-ranking but unsatisfying content with lots of ad links mixed in.