I think it's more of a "China has been known to conduct mass espionage/monitoring of its citizens and abroad. Given that known fact, TikTok poses a concern." Don't think there's hard proof they are doing nefarious things via TikTok.
> A China-based team at TikTok’s parent company, ByteDance, planned to use the TikTok app to monitor the personal location of some specific American citizens, according to materials reviewed by Forbes.