I believe the actual truth is that the tunnels providing access to the collapsed test site mountain were destroyed. The area was contaminated and unusable and the tunnels had no value.
The last test shot fractured and collapsed a “chimney” to the top of mount Mantap, so that any future test under it would have been guaranteed to “vent” radioactive gases and particles.
Not only would that annoy the big neighbors, it would yield up a treasure trove of intel to countries with the means and technology to sample and analyze the effluent...