The implication of those mainstream reviewers, which is plain, is that if a book is good, it can't be science fiction, even if it has a science fictional plot and setting. Jordison compares the two books and gives McCarthy's precedence only because he thought the last third of Miller's book didn't hold up to the quality of the first two-thirds. This assessment bothers me not at all. It is honest, and it does what all good reviews do, which is to see each book as its own thing, not merely as a representative of a category of works.
On the other hand, Michael Chabon's NY Times review of THE ROAD refuses to call the book science fiction, even while acknowledging its similarity to specific works of science fiction. But then Chabon says, "The Road is neither parable nor science fiction, however, and fundamentally it marks not a departure but a return to McCarthy's most brilliant genre work, combined in a manner we have not seen since Blood Meridian: adventure and Gothic horror... The Road seems to work its way back to the rich storytelling borderland of horror and the epic."
I can buy that! Since I define horror as any story where the author's main concern is to make the reader feel a sense of dread, I can agree that THE ROAD fits that description. And it's not like Chabon is looking down on genre; he's just arguing which one fits best.
I guess he can keep the Hugo, then.