I agree with the idea that they (the authors) haven't gone into the funding of the various witchunter backers but I'm assuming that this would have been "outside of our purview" or at least outside of that which they can test, and back up, with data in the manner they are trying to do.
I also agree that to a certain extent that is "ivory tower cladding" but perhaps they are trying to avoid leaving themselves open to claims of bias.
The door is definitely open to an investigative journalist (that doesn't much care if they burn their bridges) to combine this research with the rest of the story which I think most regular readers here already now.
A worrying thought I had as I read this was me wondering what the figures would look like for focus groups who were asked whether they had heard any of the information contained in this book. I'm thinking it would be depressingly few that said yes.