The entire Chinese statement is based on North Korean “nuclear leakage or pollution”. What does that mean?
It means the Chinese can fabricate an excuse at their convenience.
Underground tests do leak. The fallout plume is nothing like an atmospheric test, but they do leak, unless the test bore is deep enough so that there is no subsidence crater. The Chinese have thrown down the gauntlet that if a plume goes over Chinese territory, it will be considered an act of war.