The main give away that it's the plugin is that when disabled, nothing happens at all. Zero requests apart from legitimate traffic/requests.
The issue was pretty confusing to start with, when I noticed it I saw that the traffic was coming from the server the sites are hosted on, with no referring url shown (which is why I thought cron). Then digging a bit deeper I saw that the request was actually being made with the User Agent of PHP5... rather than a browser name, botname or feed name.
It was only at this point I considered looking at the plugins in turn, and disabling this one stopped the requests.
I agree that the 20min intervals is a very weird part of it, especially with no external request being made. The only thing I can think of at the moment is that maybe WP cron is giving it a kick at those intervals, maybe as a side effect of it doing something else.
Alternatively, the code itself may be looping.
A problem like this I would have expected more people to pick up on, and if it wasn't for me noticing this on 2 different setups (but same issue) I would have chalked it up to the setup.
I'd just like to say the functionality of this plugin is really good and much needed, but while providing the bandwidth overhead - I can't use it.