THAAD is useless against NK. It was just an excuse.Likely Korean cars were not cost competitive in the Chinese market in the segment they were targeting, and their automakers had to downgrade the cars. THAAD was a reaction to NK, SK has to prioritize its own national security and THAAD won't not be able to target Chinese missiles en route to the US. Not sure why Samsung wouldn't replace phones for Chinese users though. That seems very shortsighted and probably contributed to Samsung being outcompeted out of the Chinese mobile market.
The threat from THAAD is the tracking of ICBM fired from China. In the scenario of a surprise first strike by the US against the small Chinese ICBM arsenal, majority of the missiles get wipeout. The few surviving ones fire at CONUS will be tracked by THAAD early on, increasing the interception probability of the NMD base in Alaska. There is a good chance none of them will reach CONUS.