Am I reading this right? Are the hugely knowledgeable (and most welcome) posters saying that the subsurface pinger signal may be subject to range extending effects just as HF radio waves are bounced/skipped off the ionosphere?
No. At the high ultrasonic frequency used by the pinger, attenuation with distance is very significant, so the signal at detectable levels just doesn't go very far.
At much lower frequencies, where passive sonar is used for submarine detection and tracking, signals can travel hundreds or even thousands of miles. They are refracted by the thermocline (an area of rapid vertical temperature change) a few hundred feet below the surface. In that sense, it is similar to tropospheric ducting of radio waves and perhaps a little analogous to HF ionospheric "skip" propagation.