What higher frequency? The only reference in the article is to MHz range frequencies.. There is a massive difference between 1Mhz and 999Mhz, so the range listed is vague at best. The idea for wireless power transfer is to transfer power over a distance with as few other objects in the area interacting. Keep in mind in the Mhz range ANY conductive object with a length that is at harmonic equivalent to the wavelength of the oscillator frequency will receive power in direct relation to the harmonic it's naturally resonant at and that of the transmitter. Higher transmits better, but in the case of wireless power this is BAD because a power transmitter of sufficient energy at higher frequencies will naturally broadcast, oftene into other devices, such that sufficient power can be delivered at higher frequencies to electrically fry other devices not designed for it.
Also keep mind the study of human anatomy has never been done with any wireless power setups and that there are a multitude of naturally resonant structures in the human body.