Can't agree with that
Sabbath's 1st album wasn't until 1970, After Led Zeppelin had 3.
Communication Breakdown, Dazed and Confused, Whole Lotta Love, Heartbreaker, the Immigrant Song (and probably a few others) can be considered Heavy Metal and preceded Sabbath.
You need to start working out because none of those songs are heavy metal. Sure, that's some pretty hard rock right there, but that's not metal.
One thing everyone is missing with respect to metal isn't power chords or distortion. Try the devil's tritone. Sabbath did it first. (Well, if we don't include classical music) It set their style of hard rock apart from others, and that aesthetic created an entirely new genre.