It was actually tor (the threat came from tor), and harvard 'found' him by constantly logging what connections were going to known tor entries from on campus. As it turns out he was one or possibly the only one using tor that morning from harvard.
Bruce outlines it that he certainly could have stayed tight-lipped (all evidence was circumstantial) but, nevertheless confessed as soon as they approached him.
I'm looking forward to when VPNs always throw up chaff traffic.
Correlation attacks are a bitch and i'm sure i'm on a shortlist already but calling a politician an idiot with a burner account made using a vpn should be fine.