The comment on the Astute is more to do with the processing capability of the sonar rather than its sensitivity.

Very simplistically in any body of water, of sufficient depth, there are different layers deliniated by water temperature as anyone who’s read a Tom Clancy novel is likely well aware. In oceanic waters there is a condition known as the deep sound channel. This can carry acoustic signals over oceanic distances – whales use this to communicate with each other over vast distance – so, if you trail a sensitive array in the channel and have the processing power to isolate discrete signals amongst the background you can make a detection.

Getting a good value for range on that kind of contact is very hard. Triangulation is a swine when you have one leg thats 3000nm long!. You really do have to motor to get a worthwhile baseline!. So you aren’t tracking anything, but, it makes a great soundbyte saying that the sub can detect the QE2 on the other side of the Atlantic!.