This is different though.
It looks like they are using transcranial stimulation, i.e. putting electrodes on both sides of your head and running a current through it, resulting in what I assume to be a quite homogeneous current distribution throughout brain tissue.
RF waves' tissue penetration is stronger at the surface of a conductor, see Skin Effect.
EDIT: substituting the frequency in [1], we arrive at ~9mm skin depth for 918MHz.