Also, there could be a risk of ear damage if a non-linear material came into the sound field, and producing distortion artifacts in the audible frequency range - given that about 1W/m^2 can be enough to cause damage.
My dentist's ultrasonic dental plaque removing tool produces audible squeals reminiscent of analog tuning of an AM radio. That's one possible example of probable nonlinearities (in my gums an teeth) producing detectible audible power from ultrasonic energy transmission. No reference -- it's just what I hear every time I have my teeth cleaned.
Granted an ultrasonic water stream (dental application) has different physical characteristics than an ultrasonic air pressure wave (ultrasound).
Ultrasound is mechanically filtered, so it can't propagate into your cochlea and damage the hair cells. This is in contrast to something like photons, where UV or infrared light can absolutely damage your photoreceptors in your eye.
50KHz is just two octaves above our normal range of hearing. She's claiming "charge laptop in 3.5 hours" levels of power transferred to a transducer "the size of a flash drive" a useful distance away.
Now apply the inverse square law to estimate the necessary output power or assume a focused beam.
Now imagine that flash drive is your ear, your eyeballs, or a pregnant lady.