Ships this large would be carrying enough people that out of six boats sunk, somebody would have been able to swim away. This says to me either the Ceylonese navy is being thorough and making sure there are no survivors; or the North Koreans are committing suicide rather then be captured.
Thinking back on the various incidents with North Korean smugglers being sunk by Australia, Japan, and South Korea, I don’t ever recall hearing of a survivor.