I don't mean to pick a nit, but from looking at the questions and the results, it seems that the study is quite flawed.
A big part of the problem is that the questions are more weighted to 'church activity' than to questions of orthodox faith. One could answer "yes" to questions 1,3,4,5,6 & 9 (and even 2 & 11) and still not fit the criteria as "post-christian." Lest that seems far fetched, I know of several in PCUSA churches, who faithfully attend and serve, yet deny the Christian faith and have no remaining remnant of anything that could be considered "Christian." Whereas there are others, who do not attend, serve, or place any importance on prayer and the scripture and, while they could not be considered Christian in the normative sense, still acknowledge basic Christian truths.
A couple of cities listed here I would consider strongly Christian (ex: Greenville SC & Grand Rapids). While evidently there is a great number of unchurced, those who are faithful are far more likely to attend a sound, faithful, Bible-believing church (as opposed to other places like New Orleans or Las Vegas).
Appartently consideration is not made as to whether a person attends a Roman Catholic, PCUSA, or ultra-liberal church or something that is a lot more faithful. And that constitutes a serious flaw, although not a surprising one from this source.