This isn't a First Amendment issue, it's regulating commerce with a foreign nation.
When the federal government set limits on crop production with the Constitution's Interstate Commerce Clause as its justification, Roscoe Filburn was simply growing wheat over the limit to feed his farm animals. That wheat was never sold, and it never crossed the property line to leave his farm, much less crossed state lines. The government still fined him and he lost his case in SCOTUS establishing precedent in Wickard v. Filburn, because it affected the market prices of wheat, despite the miniscule impact.
The same could be said of TikTok even if it doesn't earn a penny in profit.
(Note that I am sympathetic to the idea that TikTok is a source of foreign influence. But it's not clear to me what precedent allows the US congress to control their ownership without doing the same to every "US" corporation that's incorporated in Ireland.)