Surely the issue is that if the TikTok app has access to users' address books, and can map networks of individuals and monitor their behaviour. I would expect even obfuscated identities can be identified by their networks of associated identities in their address books - for instance, lets pretend I have an anonymised TikTok id, but my address book lists my mum and dad's numbers, and my kids numbers - it's not going to be difficult to work out my true identity.
Potentially the TikTok app can push content to individuals to influence core-beliefs (of course, this happens already with current algorithms to push content to keep you looking) it's not a big leap to go from pushing content for a specific demographic to pushing content to a specific individual.
Or, even more insidiously, to report back the sort of content individuals are watching, perhaps providing an opportunity for foreign actors to compromise influential individuals binge-watching TikTok content that is inappropriate to their demographic.
Does TikTok do this already? I've no idea. Could it do this? Yes absolutely. Why would TikTok do this? Make money through advertising, de-stabilise western politics and economies, gain access to intellectual-property, or bribery and corruption - maybe? Why take the risk when you could more easily control it or just close it down.