I *think* the main national security argument is that TikTok can tweak the algorithm to incite discord and inflame conflicts within the US. They can do this far more effectively from their own platform than by doing it on other social networks via bots and fake accounts.
I tend to come down with the same opinion as you though. It's all still essentially a first amendment issue and the negatives I listed aren't going to disappear if TikTok goes away.
Yeah, unfortunately "inciting discord" and "inflaming conflicts" is very well protected by the First Amendment. It's the entire negative consequence of truly allowing free speech (not counting movie theater fire exceptions). I guess we're gonna throw that out all on a whim in 2 weeks...
I tend to come down with the same opinion as you though. It's all still essentially a first amendment issue and the negatives I listed aren't going to disappear if TikTok goes away.