THAAD has two modes. Forward-based and terminal. Forward-based mode is useless for terminal defense because of the extremely slow scanning rate. But it has a massive range in that mode. AFAIK China and South Korea had a deal to only have the TPY-2 at terminal mode. China's ESM assets can easily verify which mode it is working at. As long as it faces NK and is in terminal mode it is not a problem. If not starting selling some commercial equipment to North Korea would be a good statement.