If you look at the case, I cannot imagine how SCOTUS could have ruled any other way. The government actually tried to argue that they had to power to censor a book, even if that book had one line that they deemed to be advocating for a candidate [1]. Worse, the FEC standard was arbitrary and capricious. They censored an anti-Hillary documentary put out by a conservative group (Citizens United) but did nothing against Moore's Fahrenheit 9/11 (which was expressly put out to advocate against Bush).

[1] https://en.wikipedia.org/wiki/Citizens_United_v._FEC#Argumen...

"During the original oral argument, Deputy Solicitor General Malcolm L. Stewart (representing the FEC) argued that under Austin v. Michigan Chamber of Commerce, the government would have the power to ban books if those books contained even one sentence expressly advocating the election or defeat of a candidate and were published or distributed by a corporation or labor union. In response to this line of questioning, Stewart further argued that under Austin the government could ban the digital distribution of political books over the Amazon Kindle or prevent a union from hiring an author to write a political book."