[ChrisRR]

If I understand this correctly, the phone cannot tell if the audio is outside of human hearing range. The point of the LPF is to filter out all audio that is outside of that range.

The attack they are using is transmitting the audio at a high frequency, that when detected by the microphone generates harmonics that are within the normal range and can pass through the filter. By the time the audio signal gets to the processor, it is within audible range.