Because this is not about the first amendment? This just happens to be a company that runs a social network. Congress regulates commerce with foreign nations and made the decision, as it has in many other cases, that a foreign nation can not be the beneficial owner of TikTok. TikTok then made no effort to divest, giving away the game if you want, and predictably lost this challenge.
Well, yes, because that is the only hope TikTok had - to claim it was targeted because of the speech on TikTok, and not because this is a very boring case of regulating commerce, which as said is well established and has lots of precedent. And their expensive lawyers made it happen, when they should have been looking for buyers. And then SCOTUS unanimously said nah.
SCOTUS fully agreed that the law violates the First Amendment as written, it wasn't even a question at any level from the district court on up.
The decision was balanced on strict or intermediate scrutiny. At the distict court level it was observed that the case should probably be decided via intermediate scrutiny, but they upheld the ban under strict scrutiny due to "national security concerns".
The SCOTUS didn't bother with strict scrutiny or national security, and decided that the correct analysis was intermediate scrutiny and that the ban merely needed to serve a compelling government interest (which regulation of applications controlled by foreign adversaries meets).
It's entirely about speech, the only question in the entire case as decided at the district and SCOTUS level was speech. Whether the government should be allowed to violate the 1st Amendment due to compelling interest is everything the case turns on.
Personally, I think using intermediate scrutiny here is wild.
The arguments presented to the SCOTUS and the opinion itself are totally contained within the context of the First Amendment. No one is even arguing about anything other than the First Amendment and the exceptions permitted to that amendment.