Not so sure I agree with you on that front. In Falwell, the Supreme Court focused on the fact that the fake “ad” for Campari that was the parody making fun of Falwell (alleging he had a drunken incestuous relationship with his mother in an outhouse) had a disclaimer at the bottom clearly stating that the “ad” should not be taken seriously, and was not, in fact, real.
If you watch the video of Colbert, it seems that he actually moves from satire early in the monologue to a political commentary where he’s knowingly and maliciously using falsehoods to defame the character of the President. Around the 10:30 mark, when things get salty, all real semblance of satire seems gone from the speech—including where Colbert says he’s not afraid to insult the President to his face.
The decision noted that fact, but the decision was not focused on that fact, and the decision does not hinge or depend on that fact, not in the least. The decision spends much more time on the notion of "offensive," and concludes that "offensive" is what the 1st amendment protects.
Colbert is nothing more than a live version of political cartoon.