Sometimes, public opinion can be swayed very easily, by igniting the first spark with something outrageous; this is especially fruitful in times where the president of the United States openly opposes journalism, spreads lies, and generally fosters distrust and doubt. Lots of people are more inclined to believe a random TikTok than a professional journalist with decades of experience; what do you think were to happen if the Chinese government sees immediate value in the US government making a specific decision to their benefit, and one of the tools in their toolbox is playing a flurry of short videos to millions of American citizens, made to influence their understanding of an issue?
Most people will follow a reasonable opinion if it's the first time they're confronted with a complex situation. TiktTok is the perfect tool to exploit this, by delivering this opinion to absurdly narrow target groups, in a matter of seconds. Just because you don't notice this right now does neither mean the capability doesn't exist nor that it isn't already happening—which may be one of the reasons there is a bipartisan effort to pull through.
and that makes it different from IG, Facebook, X, YouTube (etc etc) how exactly?
With all due criticism, there are still checks and balances in place in the US that make it a very different place. We're not talking about an objectively "correct" decision here, but what is in the best interest of the USA and its allies, and that certainly makes a difference when it comes to foreign influence on the own populace.
All that being said: American Tech companies are dangerous in their own right, and nothing in my post was defending these either. But that doesn't make TikTok less of a threat.