It definitely straddles the line between several genres, for sure. And you’re right; it’s not a straight up horror game per se. Still, I think it’s horror/action ratio is quite similar to, say, modern Resident Evil. (it’s got that zombie/infected vibe going on.)
But, yeah, when I think of horror games it’s not one that jumps into my mind straightaway. (That’s probably why I overlooked it.)