I'm on mobile so I apologize for any misspellings or formatting, but I think it's important to bring the transcript of DC's comments on this here because if you hear/read his words I don't see how you can still question that The Shack is code for any evidentiary element to the crime.
From Scene of the Crime, A New Direction, starting at 50:10.
Narrator: After the press conference, online sleuths went into overdrive, questioning whether Carter's references to The Shack could be a coded message to the killer, or perhaps a confirmation that Abby and Libby's deaths has involved a shack....On our behalf, KG asked Superintendent Carter about his reference to The Shack.
KG: Another thing..."The Shack" was used in it (note: "it" is the April 2019 press conference). Was that on purpose?
DC: It was. Have you seen "The Shack?"
KG: I have. I watched it twice.
DC: Then you know what I mean.
KG: Yeah. So I think, one of the big rumors going around is that they died in a shack or that there has to be, you know, some other meaning than faith to it, which is something (note: "something" is the rumor) that we were trying to, kind of, get rid of.
DC: There was no shack involved. But that had a really, really, uh, compelling effect on me.
KG: Yeah, I think that movie in general - the first time I watched it, it had zero effect on me. I kind of sat through it and watched it and saw no meaning behind it because I just wanted to watch it as a movie. Then the second time I went back and watched it I cried the entire movie....
DC: I have to say, it had the same effect on me. In a different way. Because, you know, I don't understand what it's like to lose a sister but that movie had such a profound effect on everything about our life, that it was worth mentioning.