One of the early Heavy Metal bands:

[JohnnyRingo]

12. Metal is hard to define. All my dad heard was racket from my Sansui

I believe it takes more than a guitar shred to qualify as Heavy Metal. Should be more about edginess and attitude, two hard to define qualities.
I consider Black Sabbath a dark hard rock. Much of their work is melodic ballads with great guitar solos. Closer to heavy metal than Iron Butterfly though. Still they all had songs that fit the general bill.

I don't think you're wrong because metal, hard rock, and punk are all hard to pin down.

I think this is an example of that Heavy Metal attitude and edge: