You got all the details right but didn't listen to what she said.
- Yes, Sarah Vowell is profoundly liberal.
- Yes, she's not a believer (in fact, she's essentially turned her back on church/God, which she does get into at other places in the book).
- Yes, she thinks the CCM mid-tempo pop ballad is insipid (she's not alone on that one).
- Yes, she wants music that feels root-sy & authentic...
...which is her point (and mine). We in the church often assume that we KNOW what seekers want - but we don't actually listen to seekers to find out. Much of what the emerging church has done right is because they've tapped into the authentic desires of folks seeking a real & vibrant faith, rather than simply creating our own Christian subculture hermtically sealed against corruption.
But I've gone off-topic - you questioned the logic of her statement. Being a political liberal does not define your musical tastes, any more than being a theological conservative does. What she expressed is a desire for gospel music - that to her, like many of the senior adults in my congregation, there's a particular sound that rings true to them as "professing Christ".
I'm not saying that every church should do Johnny Cash's "Man in Black" each Sunday (though it wouldn't hurt if we played One Bad Pig's cover of it every once in a while). :-) But I think we must listen carefully to those in the culture around us to hear how our attempts at communicating the most important message ever is coming across. Not so we can change the message (cue dope slap) but so we can make sure the way we're communicating doesn't speak louder than the truth of Jesus Christ.