The problem is as soon as you're speaking of "frequency" you're already on a time-scale where the power doesn't interfere with other frequencies (as long as the medium is linear, etc -- which is almost certainly true here). In the mathematical/technical sense (in linear system theory)it's quite clear that different frequencies would interfere, it's a basic entire principle of linear circuit design that's been used for centuries -- systems have a Frequency Response where each frequency is independent as a result of the eigenfunction property. Personally I just think it's a big oversight, maybe they were trying to give a laymen explanation and confused themselves. What they're doing must be some form of phase modulation, maybe they're interpreting as short bursts of frequency modulation (with a stable steady-state frequency), but as I said this is not an illuminating view imo.

Edit: Actually I think they might have frequency control indeed. That would be a good idea to find a peak in frequency response (i.e. "resonant power transfer"), not for any directivity gains through constructive interference. That might be the source of confusion.