The source is Word of Jim. He didn't literally say that, but he has referred to both Justin and Harry between Changes and GS as D-E-D dead.
I recall the Justin one, but not that he used the same phrase to describe Harry. I thought the Word of Jim mentioned in the OP was Jim flat out saying "Harry is as dead as Justin," which puts a different spin on it.
I'll take you at your word that he used the same phrase.
I've been pretty sure that Justin's still alive anyway. There's just no way that Harry doesn't get answers by the end of things. Personally, I think that Mirror Mirror will reveal that Justin's alive over there, which freaks Harry out, so he goes looking for him in the Prime universe.
I'm not entirely sold on Cowl being Justin, but I wouldn't be surprised if that was the case. It's one of several possibilities I entertain. It has a certain symmetry, especially with the Outsider influence in White Night. And, you know, the White Court shenanigans taking place with He Who Walks Behind was kind of a cluebat.