I read the PDF (shock).
It sounds suspiciously like they just checked the logs to see who had visited Tor related websites and then went and interviewed the handful of people who happened to visit these sites within a few days. Maybe interview those who had exams in the 4 listed buildings at the designated time?
Or, possibly, they just checked who had used Tor in the last few days on their network - can you ID a Tor packet by looking at it?
It doesn't sound like they needed to crack Tor.