Ever see the waterfall displays of passive sonar? I would not be surprised if terrain could make a void in the background noise which would be detectable if you were configured to look for it, and that configuration would probably involve running slower.
I've never served on a sub but I know a thing or two about electronics and what is feasible.
You may be correct. My comments are based on limited knowledge. I could see that terrain would mask background noise and be “visible” through that method. Seems you may be correct since they just axed the command.