Tip-off from another underwater source?
I hope this 37.5 signal is confirmed and leads to discovery of the plane. Scepticism seems churlish and unfriendly at this point.
The doubts with regard to range of the signal may not take into account the possibility of other floating wreckage having been spotted, or the signal having been detected initially by some other unknown equipment on/deployed from chinese ships, leading to to this vessel being sent to the location to confirm the detection.