(They fail to follow through with posts on G+ are naturally likely to get more +1s than likes, thus presumably debunking the whole correlation issue like you say. But anyway)
So my takeaway (which I am not sure if you are challenging) is that posts on plus.google did better than a presumably control sample on facebook - and the article incorrectly ascribes this to +1s. You seem to be saying, yes it did do better but its not because of +1s, its just better. But is it better content or is it better SEO-ness of the page.
Thats the part I would like to see these studies show - how they manage the control portion. How they control for quality of the content? Because if posting the same content on G+ and on a.n.other site gets you significantly more pagerank, then its really hard to argue not to do that.
(I quite accept the "its not +1's goddammit" argument)