When metal truly began is definitely a topic of some debate, but Sabbath's debut has by far best argument imo. There are without question elements of what we now consider to be metal in a ton of 60s songs, like bits and pieces here and there where we can hear some eventual metal tropes and stuff. And I guess Born To Be Wild could be seen as one of those examples, as are a couple of Zeppelin songs and a couple Iron Butterfly songs, Blue Cheer, The Kinks' early stuff, Cream, so on and so forth.
But a big part of what makes Metal, Metal is that it discarded the Blues Rock roots and became something relatively new entirely. And pretty much all of those bands and songs listed above are way too steeped into Blues Rock and Psychedelic Rock to fit into the definition of Heavy Metal. They lack the goth, dark atmosphere that Sabbath pioneered and that defines the sound of the genre. Admittedly Sabbath's debut has some bluesy parts at points as well, but they also reshaped it into something vulgar and ugly in a way that hadn't been done at all before. And by Paranoid and (especially) by Master of Reality, they had pretty much done away with it completely.