My guess is that the vessel was not involved at all in the test of a DF-21D. It sank, for whatever reason, in shallow waters according to report, while in tow. I do not believe that would be the profile for such a test.
In addition, other nations, the US inparticular would have detected such a test and, if there had been no advanced warning, would have protested it.
The US this week actually did test a hypersonic strike wepaon that flew to and hit its target on an atoll in the Pacific, flying 2300 miles in less than 30 minutes at over Mach 5. It was well announced and open about it. I expect when the PRC tests the DF-21D in a live test into the Ocean, and probably well out to Sea somewhere, they will let others know and establish some kind of zone around the target area.