It sounds like the nuclear testing site became unstable.
So they are abandoning that as their testing site. And they may not have another suitable site. In which case they may have to abandon testing altogether.
That’s not to say they are abandoning their arsenal.
They may all or part of their arsenal up for negotiation to get relief from sanctions. And by that I mean, they may keep some of their arsenal while pretending to give it all up.
Believe I read that Chinese geologists said it was collapsing already, so "closing" it may be a meaningless gesture.