Well, the TikTok lawyers kinda said the quiet part out loud during their SCOTUS brief:
Mr. Francisco contended that the government in a free country “has no valid interest in preventing foreign propaganda” and cannot constitutionally try to keep Americans from being “persuaded by Chinese misinformation.” That is targeting the content of speech, which the First Amendment does not permit, he said.
It's not a great look for your app when your argument before the Supreme Court is "yeah, we're a propaganda machine for a hostile foreign power, but free speech says you can't stop us. Neener neener."