The news is in Korean yet.
Letting PLA to use Port Najin will make PLAN running freely near both South Korea and Japan.
previously it was a purely commercial trade lease .. recent regional changes make it necessary to establish a naval base to avoid being boxed in the yellow sea bottleneck ... overall policy change seems that in the future the chinese will establish overseas bases to secure its trade routes, it may change the no interference policy (which generally doesn't do anything for chinese interest in reality and is actually a handicap) to face the reality of international politics.
in any case no way to be sure this deal is even real or just media spin, only if there are PLAN ships stationed at Najin, then we will know for sure
If we see the PRC's position hardening against sanctions on NK again, now we know why
it hardens its position is a recent development, its support of the new kim dynasty is in light of changes in the geopolitical region... a year or two ago the possibility of the acceptance of a unified korea on the south's terms might have been possible as a deal between the PRC and south korea.. with the change of a more hardline government in the south and US moves to include both the south and japan in so sort of union aimed at china have more than likely killed that possibility and thus the PRC is now backing the NK as a base of operations as a hedge.
previously sanctions would be watered down to avoid boxing the north into a corner where china's very limited influence would no work and thus cause the regional situation to spiral out of anyone's control, they want to keep the US in check , not have a regional war. Now the calculations have changed into using NK as a passive proxy to an active one