Really early heavy metal and Hard rock were the same thing. The sound grew more extreme and evolved to many places after that, but in reality early hard rock, and metal were basically the same thing. Some people just have a stigma against the label heavy metal and thus, said it was not them. Yet they were often heavier than people that were considered metal, and they still claimed they were hard rock.

So during this border period where rock was becoming something heavier. That is where metal was born. :)