Yes, the cost of benefits has increased, but the study indicates that the cost of benefits increased from an average of 25% of faculty salaries in 2001-02 to 35% in 2016-17. So, yes, a “driver” of overall cost increases, but a fairly small one. At public universities, faculty compensation (salary + benefits) typically represents about 30% of total costs, so the fairly dramatic increase in the cost of faculty benefits over that period would represent a 2.4% increase in overall costs.
Actually, it is clear. The study indicates that it’s based on full-time faculty salaries. Adjuncts aren’t considered full-time faculty. And it’s standard practice for full-time assistant, associate, and full professors to be on “9 month appointments,” on the assumption they’re being paid for two semesters or three quarters of teaching per year—even though in most cases they’re also expected to produce research that will have them working year-round.