Simultaneously, TikTok has captured the daily attention spans of millions of Americans, many of them especially young and impressionable. They've captured their attention in the form of a black box algorithm that promotes content in whichever way TikTok deems most appropriate.
These two facts mean that the Chinese government now has direct access to the brains of millions of young Americans, with zero oversight. Imagine China subtly promoting videos to create outrage and civil unrest in the US, or to feed anti-US propaganda to germinate terrorist groups in the US. These might sound like far flung possibilities, but I think it's hard to say what a country that views the US as its enemy (politically, economically, philosophically) might do with that kind of power, and I don't think we can wait to find out.