Seems to me it's only guilty of the same data scraping for marketing purposes that Meta is... maybe even less so as I always found TikToks advertising less gross than Instagrams.
Both suck but TikTok is slightly less suck.
Is it really a security thing or is it a "We are getting pressure from US companies to remove competition" thing?
Similar to what others said, it is about control. The US state has complete control over how the "algorithm" feeds people the narratives that the US wants to feed them when it comes to the major social media platforms. But not with TikTok. It is not a national security threat in the way they want people to think it is, but it is a national security threat in that they can't force their curated propaganda through it as they see fit, like they can with the other platforms. It's a propaganda liability.