A light comedy piece or a plucky human interest story do not erase the statements of fact made or the repeated insistence on being taken seriously which pervade the rest of this institution. It isn't even reliant on their audience taking them seriously, it's reliant on the intended tone and how a reasonable person would perceive that intent.

You can argue that Fox News is intended to be basically the Colbert Report satirizing a certain mindset, but it's an obviously bad-faith argument. The Colbert Report was literally created to satirize the seriousness and mendacity of Fox News and its attempts to persuade people into a set of not just interpretations of the world, but factual beliefs about that world.

There is a line, and Fox runs way over the line into defamatory content multiple times an hour.

I can't immunize myself from currency counterfeiting charges by claiming that I never thought the copies were real, that it was all just in fun, that I was pranking the businesses I spent them at, and that my Youtube channel includes other fun bits of me deceiving people and telling jokes. The one does not exculpate the other.