I made clear at the time that the study claimed 80 percent of traffic to Tor hidden services related to child pornography, not 80 percent of all Tor traffic.
This is also a bogus number. An estimated 80% of Tor hidden service directory lookups are for those sites. Long-lived connections only make one lookup, and sites that people check multiple times in one session only get looked up periodically. The ones that are looked up the most often are the ones where people open Tor, visit the site, then close Tor. This does not correlate with volume of Tor traffic at all.