Only DC is not accountable to us, the laymen. He was addressing his monologue "directly to the killer". (Probably, also taking into consideration the feelings of the families). So whatever he said at that moment, and however he interpreted it later for the public, might still differ.
In a way, knowing that DC is very religious, i believe he could have achieved several goals by mentioning the film. One, giving some pragmatic hint "directly to the killer". Two, appealing to whatever shreds of religious consciousness the killer, per DC, might still have. Three, perhaps consoling the families and the citizens of Delphi by alluding to "the Shack". Four, spreading the knowledge about the movie among broader public who might have been unaware of it.
I think all of it could be true.
However, what slightly bothers me is the fact that DC views the killer as religious one. What if the man is not, or is "losing his religion". Could ISP be making an error here, a type II error, and cutting off a whole group?