Thaad is used to :-
(1) intercept nuclear missiles if fired by N Korea
(2) serve as deterrent to prevent N Korea from firing - because if intercept over N Korea territory, nuclear will explode in N Korea rather than at other places
As Thaad radar is too long range and intrude into China territory, S Korea can work with US to isolate the range of radar into N Korea and not go over China's border.
This may alleviate China's concern that Thaad is compromising China security - and maybe able to get China to buy-in to Thaad deployment.