They weren't even dissing FG the show though, they were dissing the lame McCain-Palin joke that fell flat. Honestly, very few people on the discussion thread for that episode found that funny, why shouldn't Matt and Trey diss it?
That's why I love South Park; far too many shows are politically partisan (especially coming from the liberal camp, as shown by the aforementioned FG gag), but South Park disses, praises, and exploits in weird situations such as an Oceans Eleven parody both sides of the politcal spectrum. Really, it's a shame there aren't more shows that take this direction...