I assume you're being pedantic, because the answer to your question is obvious, but I'll explain what he meant anyway. Once the data is in China, in the hands of the Chinese government, their use of it might theoretically still be "subject" to US law, but as a practical matter, nobody could do anything to stop them from doing anything they wanted with it.

But what is this "whatever they want with it" that would be illegal in the US?

I was just explaining the poster's point. Personally, I think that most data produced/collected by social networking applications is worthless. All of these draconian privacy laws that have been passed, in my opinion, are nothing more than political theater and make little to no difference to 99.99% of the population. GDPR, for example, has produced nothing more than lots of annoying popups that nobody pays attention to, and billions of dollars in fees for compliance consultants. I can understand the government banning people that have security clearances from using certain apps, but blanket banning an app seems like an overreach.