I was wondering how this could make sense until:

The result is an antenna that operates at very low frequencies, around 35–36 kHz, while remaining far more compact than the conventional electrical antennas that work at those same frequencies.

They are using a super low frequency.

Very low frequency radio waves is the traditional means of communication with military submarines, while submerged.

However, this required huge antennas and very high power transmitters, so this was used mainly to transmit short messages from a terrestrial station to submarines, for instance instructing them to send an antenna to the surface, for bidirectional communication at high speed.

The innovation here is the use of a new kind of antenna, which can work well under water despite small dimensions, and with which a low-power transmitter is sufficient for communication with other submarines or with a surface boat, up to a few hundred meters.

Thank Shannon!