True, which is kind of scary, but in the same paper they use a simple padding technique as a countermeasure and the detection rate drops to 3%. There are known, effective defenses, but they haven't been implemented on the live Tor network, mainly for performance reasons. The Tor folks choose usability over absolute safety in a lot of design decisions. I guess the rationale is that these attacks are unlikely to be occurring, and it's more important to get a lot of people to use Tor (which itself enhances security).