(I'm the author). Your last point is very true, and the chart tries to make it very clear it's only talking about public posts. Obviously entirely true that they could be using it purely privately, but I still think is an observation worth making that they're not using it publicly.

As far the correlation in general, as I mention in the thread above, these aren't normal people, they're the leaders/overseers of the company using (or not) a landmark initiative whose leader (Vic Gundotra) clearly thinks that ideally it'd be used for both private and public interactions. At the very least, it'd look a lot better if they had accounts, tried making a public post (even just to congratulate the team publicly), and looked a bit more publicly engaged.