Who said it is supposed to be a metal album? People on any music site who define what genre is an album?
I don't think many artists/bands really care, if they play metal, or not, or how you define metal.
Of course, Metallica doesn't care, in particular, though that's more recently as in the 1980's they really cared about what they did. As their first album was supposedly titled "metal up your ass" but the recording company didn't allow it.