"For the above idea to work with unmodified off-the-shelf microphones, two assumptions need validation. (1) The diaphragm of the microphone should exhibit some sensitivity at the high-end frequencies (> 30kHz). If the diaphragm does not vibrate at such frequencies, there is no opportunity for non-linear mixing of signals."
The devices tested include hearing aids, smartphones, smart watches, etc, which are all likely to include small surface mount MEMS microphones. I doubt any of these techniques will work against a larger dynamic or condenser microphone, where the mass of the diaphragm makes the system inherently insensitive to ultrasonic frequencies. There's a reason the jamming signal is inaudible, and it's not because our auditory cortex contains an ideal lowpass filter.