Who said what in what interview whenever is irrelevant really (although I've seen several interviews with Geezer where he credits Sabbath with inventing metal via old horror films influencing their early tunes), what it comes down to is the music.
AC/DC's music is hard rock personified. There is nothing metal about any of their music, it is pure blues-based rock & roll.
Maiden & Sabbath on the other hand, are different. Listen to "Into the Void", Symptom of the Universe", and of course the song "Black Sabbath", and Maidens "Aces High", Number of the Beast" and so on. Sabbath's heavy chugging riffs and Maidens intricate guitar harmonies and galloped rhythms are undeniably metal, whereas AC/DC don't have those qualities in any of their music.