This case was not about speech. It was about a vehicle for speech having a high risk of being used for espionage and PSYOPS. If TikTok was the only vehicle available for people to post on the internet, then maybe the First Amendment argument would hold water.

This decision doesn't tell people they can't speak any more than, say, shutting down a specific TV station or newspaper which has been used for money laundering or which is broadcasting obscene content.

The case is entirely about speech, and the various levels of scrutiny that apply to laws that violate the First Amendment. You should read the decision before commenting on what was argued and decided in said decision.