It's clearly a response to people getting unfiltered news from Tiktok that it otherwise not presented on mainstream media at all. It's why the ban was in the defense spending bill last week. Even outside the current Middle East conflict, the Ohio rderailment last year was extensively covered on Tiktok for 1-2 weeks before it really got any mainstream coverage.
The data privacy issue is a lie because Congress could've passed a comprehensive data privacy bill but didn't (and won't).
But the more compelling argument, and why I'm torn, is reciprocity of access to the Chinese market. Facebook, Instagram, Google, Youtube, etc are actually or effectively banned in mainland China. China's entry into the WTO sort of allows this and I understand why China does it but if the US took the position that access to the US market requires reciprocal access to the other country's market, that would be a position hard to argue against. I'm surprised that wasn't the argument made.
Tech giants have been very effective at arguing the "algorithm" decides what's shown to users and what isn't but the algorithm is merely a vessel for human decisions on what is a feature, what is ranked, etc. Facebook and IG manipulate their ranking too but they do it in a way compatible with US government policy so it isn't an issue. That's the real problem with Tiktok, as far as Congress is concerned.