While I don't think South Park is free of blame when it comes to political apathy, it's important not to ignore politics itself as at least a partial driver of that.
I'm 33, and the first election I voted for was for Obama. I've voted for every election since, and will vote for whoever gets the democratic nom in 2020, whether it's Elizabeth Warren, Michael Bloomberg, or a random golden retriever that joins the race. I care about the political process, but I can also understand that don't. And if you asked me for all the reasons why they may be indifferent, South Park would probably be towards the bottom of the list.
There will always be people who think they're above politics. And while I'm sure some of those are SP fans, the people in my life I know that always boil down their argument to "Eh, they're both the same" are people who are a decade or so older than me and, to the best of my knowledge, aren't especially big fans of SP.
Anecdotal, I know. But while I agree with the tweet in question - that the show has remained comfortably smug in both sides-ing issues and seeing sincerity as the enemy - I still struggle to conjure up much anger for it given all the other things in the world that can and do inspire apathy. And that despite all these things, many young people still seem incredibly passionate and invested in progress.