Ewan at Ewan’s Corner has posted a short review of The Road (SF Signal review). In it, Ewan states that the book isn’t science fiction because, “you never find out what caused the end of the world, no aliens and little science.”
To me, that is a very ‘mechanics’ focused view of what science fiction is. It’s too narrow, at least for my tastes. Looking at other post-apocalyptic novels, Earth Abides, Alas Babylon, Lucifer’s Hammer, we see that the above definition would rule out these books as science fiction, when they clearly are.
Moving to the realm of TV and film, the original Mad Max and Jericho also fit the above description, and I believe they, too, are science fiction, of varying degrees of quality, but still science fiction. Given all of the above, is the setting alone enough to make something science fiction? I’d have to say yes. And not just because of setting. Science fiction doesn’t have to be about the science or technology in a story. That sort of thing can exist and stay in the background and a story can still be science fiction, see Slaughterhouse Five or The Handmaid’s Tale or even Children Of Men, among others. I think people get hung up on the mechanical aspects of a story because of mistaken perceptions, as most people are going to see SF as being all about science, technology, aliens, and all the other tropes you see in SF movies.
But to answer the question in the title, yes, The Road is science fiction. McCarthy may be traveling a well worn path, but his book is still SF.