Navy VLF signals RF approx 9 - 35 khz the 'pinger' is 37.5 khz audio (ultra sound)
Quite right. A bit more explanation:
Audio signals, like those from the pinger, are *sound* - they are carried by pressure waves in a fluid.
RF (radio frequency) signals are radio - they are carried by electromagnetic waves.
One kind of detector (a microphone, such as a hydrophone) picks up audio. A different kind (a radio) picks up RF.
Occasionally, the electronics in an audio system picks up RF signals - such as when a nearby transmitter garbles your stereo. But... that is very unlikely here - I'd say impossible given the equipment the Chinese are using. Also, the very low frequency radio transmissions to submarines don't transmit in 10 millisecond bursts, while the marine locators (ULB) do.