Assuming this is not a scam, my guess is that it emits sound above the Nyquist frequency of mic'ed devices with the hope that they lack a low-pass filter on the input of their ADC. Such devices would then suffer sample aliasing that would overlay the high frequency that is output by this device with whatever in-band audio that it intends to obscure. A cheap low-pass filter would defeat this and is likely there in any case in most mic'ed devices as the world has high frequency ambient noise.