Nobody should be surprised that the Chinese government has accessed, or can force access if they want to, to TikTok's user data.
But more importantly, we have learned that social media can "nudge" user behavior by curating what users see. Foreign adversaries have successfully gamed the hell out of Facebook's system that prioritizes engagement.
Now imagine the foreign adversary actually having control over the algorithm and optimize for nudging user behavior as the main goal. It's scary, and we now now it can be quite effective.