He is closing down a test site (which was probably no longer viable, anyway, having been damaged so badly by earlier nuclear tests), so why would he not capitalise on the situation? He has not promised to destroy any nuclear stockpiles.
His stated desire for the Korean peninsula to be totally denuclearised may involve there being no nuclear powers present there at all - which would involve the United States no longer having any bases or holding any exercises there. Would Trump agree to that?