Don't be fooled by talk about security or privacy. This is simple Nation State realpolitik. TikTok is a propaganda threat controlled by a non-friendly state to the US. Any other way to look at this is naive.
Is there any evidence it's been used for propaganda? I don't use TikTok a lot, but it seems very non-political (maybe it's my filter bubble). The real cesspool of hatred and madness is Facebook - but of course Congress doesn't care too much about that. As far as I remember from the previous elections the Russian bots were operating on US based platforms
If your populist parties aren't on it, they're incompetent.