ASSUME that this story is true -- for now. Personally, am willing to wait for details, either from China or elsewhere.
First, we do not know how the sub was supposedly 'trapped' by whatever it was trying to deploy.
Second, while submarines can settle on the bottom, it is not advised to do so. Do we know the size, specifically the diameter, of this boat in question? The boat in question is supposedly Type 093 which has a beam of 11 meters and the event occurred in 20 meters depth. Ideally, there should be three boat lengths between boat and sea floor.
Antisubmarine SSKs are small, quiet, and maneuverable-just the thing for operating in shallow and narrow waterways.
www.usni.org
There a submarine can maneuver in a water column of only two to three ship lengths.
That means whatever the boat was doing, it was doing it in extremely dangerous environment -- too shallow. If something went bad, and again
ASSUMING the story is true, it must have been too severe for the crew to make escape.
You guys better pray that this story is not true.