I mean, literally right here in the first page of the arguments (page 4):

> First, TikTok incorporated as a U.S. company speaking in the United States.

https://www.supremecourt.gov/oral_arguments/argument_transcr...

I think you're confused.

The company being targeted is ByteDance, not TikTok. The US government wants ByteDance to sell it's controlling position on TikTok to someone else, or else TikTok can no longer operate in the US. ByteDance is a Chinese company that is a de-facto shell corporation of China's CCP. For the 14th amendment to apply, you would need to argue that either a Chinese corporation or China's CCP would be US citizens.