I think the concept is if you own enough exit nodes and you have monitors at the backbone level you can correlate traffic with time-based attacks.

The current number of people using tor in a given time isn’t so insurmountable that you can’t throw a couple of data centers worth of VMs at The problem and they’ve had backbone monitoring for decades.

The thing is, the feds aren’t going to come knocking at your door because you are downloading movies. The MPAA figured out a long time ago that it’s a losing battle going after individual people downloading/uploading. If you were trying to use tor to hide behind doing things to harm other people, running terrorist networks and the like, there’s a reasonably good chance they could track you down if you were just using tour but they’d have to really want to do it, and that’s not going to happen for Steve’s half terabyte of CSI.