No filter has an infinitely steep cutoff slope, it's a question of the 'degree' of attenuation. It may or may not be safe in practice, but if it is it's not "by definition".

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.