The fade to black scene is not followed by the credits! (Spoilers follow.) There is a coda scene where Chris and Sarah are shown living a new life, somewhere else, where they seem happier. Sarah is back in college and her son is riding his bike around, seemingly having returned to his normal self.
Except she takes some pictures of him outside riding his bike and his face is distorted while all else is sharp — as (presumably and was implied) had happened before when she’d thought he was an impostor back in the countryside.
That still leaves open the question of her sanity and the impostorhood of her son. However, the movie ended on a quite different note than the reviewer claims because Sarah still thinks her son is an impostor, or that she’s crazy, or perhaps both, and I’m guessing from her look decides it just doesn’t matter.
As a side note, Seána Kerslake was really good in the film. I’d never seen her before in anything but will keep her name in my head.