In the article the authors mention conducting a study:
We have come to this conclusion based on an exhaustive, web-based study of everything that the 100 wealthiest US billionaires have said or done, over a 10-year period, concerning several major issues of public policy. For each billionaire we used several dozen carefully selected keywords to find all publicly available information about their specific talk or actions related to any aspect of social security, any type of taxation, or anything related to abortion, same-sex marriage, or
immigration policy.
the authors of the article have written a book, coming out soon:
Benjamin I Page, Jason Seawright, and Matthew J Lacombe are (respectively) Fulcher professor of decision making, professor of political science, and PhD candidate in political science at Northwestern University. They are co-authors of Billionaires and Stealth Politics, forthcoming in 2018 with the University of Chicago Press.
Perhaps references & the like will be found there. Looks like the book won't be shipping till early December, but you can peruse the table of contents here:
