There are a lot of sources of noxious gases that could be generated if there's was a fire on board. But usually submariners have access to oxygen or gas masks that would allow a crew survive this sort of hazard. There should be a very obvious response from China though, like a notice to air or marine traffic and rescue/recovery vessels being dispatched with their accompanying transponders.
You are very right, baffling.
Xavier was right all along about PLAN ships not having fire extinguishers! I knew the ones we saw from @by78’s screw caps were just phony plastic ones used for propaganda purposes only. The gas masks are probably halloween masks modified to look like the real thing. PLAN saved approximately 5000 RMB and is now paying in blood.