If the Chinese really did detect the 37.5 KHz signal using a hand held detector and a hydrophone on the end of a pole, it does at least explain why the signals were not recorded. It does, however, seem extraordinary that they would be using such a device from a RIB unless they had very good intelligence that there might be something there.
How many other locations had they tried previously?
The news organisations are suggesting they might have information from the military which has not been disclosed but that probably applies to all parties involved in the search.
Perhaps just a bit of initiative by the Chinese vessel and having a try with the limited equipment they had available, could not do any harm trying.