In the precedent the U.S. government was clearly trying to find the names of Americans interested in foreign propaganda. Here the U.S. government is trying to stop ownership by foreign adversaries of social media operating in the U.S. That's clearly a significant distinction.
Did you read the actual case? It established that under the 1st amendment is a right to receive information and ideas. That established precedent is clearly at odds with the government trying to ban a source of information and ideas in the US.
From the main opinion (as opposed to Brennan's concurrence; the court was unanimous):
> We conclude that the Act, as construed and applied, is unconstitutional because it requires an official act (viz., returning the reply card) as a limitation on the unfettered exercise of the addressee's First Amendment rights. As stated by Mr. Justice Holmes in Milwaukee Pub. Co. v. Burleson, 255 U. S. 407, 255 U. S. 437 (dissenting):
> We do not have here, any more than we had in Hannegan v. Esquire, Inc., 327 U. S. 146, any question concerning the extent to which Congress may classify the mail and fix the charges for its carriage. Nor do we reach the question whether the standard here applied could pass constitutional muster. Nor do we deal with the right of Customs to inspect material from abroad for contraband. We rest on the narrow ground that the addressee, in order to receive his mail, must request in writing that it be delivered. This amounts, in our judgment, to an unconstitutional abridgment of the addressee's First Amendment rights. The addressee carries an affirmative obligation which we do not think the Government may impose on him. This requirement is almost certain to have a deterrent effect, especially as respects those who have sensitive positions.
You might be confused because Brennan says what you say the court said in his concurrence, but that nothing in his concurrence is dicta. The controlling opinion lays out the dicta, and it says what I wrote, as you can see from the quotes.
