Well, keep in mind that the Swiss 50Hz do fall into the <100kHz designation.
I'm aware of it's usage for pinging currently-diving submarines, and in the slightly higher region, providing wireless bidirectional low-bandwidth comms into deep mines. The power lines do have quite some magnetic stray fields, though. Not that it's too massive; it's not more than a few (3~4) kA at most.
I am not sure how these signals come from power lines, which is what the study looks at. I would assume those are in the 50-60Hz range. (But we can use that band for communication too! https://en.wikipedia.org/wiki/Extremely_low_frequency)