The Type 093 ("Shang I" or "II") is China's most modern nuclear-powered attack (not a "Boomer", SLBM) submarine. Its complement is 100, so either it had a less than full crew, or some of the crew survived. Also, if the boat was "only" trapped for 6 hours, the failure of the oxygen system was a second failure, and actively poisoned the crew. That it got caught in a defensive anti-submarine obstacle, whose location should have been well known to Chinese submarine officers, suggests a significant navigational error by the crew or captain. There're lots of tantalizing questions whose answers China is very unlikely to reveal.