That is true, in fact directionality is actually harder to achieve in higher tones (everyone remember those mosquito ring tones that went around a little over a decade ago?) When the wavelength gets very small your ears have less and less information to distinguish which ear is receiving the signal first, other than the brute force which ear actually heard it first calculation. Distinguishing waveforms becomes near impossible at high enough frequencies. It's interesting though that a low enough frequency might also be difficult to distinguish due to there being not enough information rather than too much. If the waveform is as long as a room how much difference is there really across the size of a head.