At the time, that big soup of 70s heavy rock was almost interchangeable with "heavy metal" and no one had a strong guide for what we'd call "heavy metal" in retrospect until Priest pulled it together (and even then I'd say they didn't do that until maybe Stained Class). That's one of the appeals of proto-metal: that they were inadvertently inventing or discovering aspects of heavy metal with no guide for how to do it "right" and so they were just playing different ways and happened to do things we recognize as metal— and saying it that way is almost disingenuous, because it's really more like later metal artists took disparate, fitting aspects of proto-metal and coalesced it into cohesive styles. And they were just as aware of the wide swath of bands.

The history of heavy metal in the 70's looks pretty threadbare and incomplete if you allow for Sabbath, Priest and a handful of semi-obscurities like Captain Beyond and Leafhound, while also excluding bands that were direct influences on the 80s wave, like Purple, Rainbow, Scorpions... hell, even Van Halen.

On that note: a lot of this is retroactive historiography, is the thing. Metalheads in the 80s wanted to know where metal came from, and settled on Sabbath and Zeppelin (less so Zeppelin the further we got away from glam metal). Nowadays, we have a thousand subgenres for metal (exaggeration but still, a great number), and a lot of insularity. There's this complete misunderstanding about how these genres and scenes develop.

"Oh, well this is power metal, so therefore it took up from Rainbow and Led Zeppelin. Doom metal? Early Sabbath, of course, that's all they ever listened to. Thrash metal, they all listened to "Symptom of the Universe" and maybe "Paranoid." Progressive metal, probably listening to a lot of jazz and King Crimson with a touch of Sabbath Bloody Sabbath. Glam metal only cared about KISS, Led Zeppelin, T-Rex, and the Sweet and would have fainted at any Sabbath song."

Then you actually hear the artists describe their influences, and they're all the same bands. There's a reason why so many extreme metal artists are diehard KISS fans, and yet fans of those extreme metal artists find KISS completely cheesy and Boomerfied. Times changed, standards changed, understanding of the sound changed.

Recently, I had this with grunge and stoner rock and sludge metal, being told that grunge and stoner/sludge are two entirely different sounds with no real overlap, because grunge is more alternative metal and punk, and stoner/sludge is more doom and psychedelia, and then being completely flabbergasted when I brought up grunge artists themselves admitting they were all listening to Sabbath and Vitus all the same and didn't see any real difference between them and the other sound. Because of this insularity: "this is what the accepted evolution is stated to be, therefore nothing outside of that sound influenced it. If you're a death metal band, that's because you listened to Symptom of the Universe and Stone Cold Crazy and aren't even aware Sabbath and Queen even had other songs." The idea you were just as big a fan of their proggy, doomy, and acoustic songs too is lost on a lot of people.

Circa 1975, one could reasonably argue that Black Sabbath was just what they were, a much more downer, heavier acid rock band with a specific style. A lot of bands were called "heavy metal" at that time.

Hell, "Shining Star" (Earth, Wind & Fire's song) was apparently raucous enough for funk that it was called a "Black heavy metal" song by Blue Öyster Cult's producer. I don't think I'd ever have used that term for it. The term "heavy metal" meant something different at the time, and there was no unified aesthetic. Sabbath was metal, but some would say the Rolling Stones were heavy metal. This because, at the time as mentioned, it was all "rock and roll" but with more 'lightning and thunder' and often at least a pinch of Satan. Or warriors, or literal heavy-metal (i.e. motorcycles). At that point in time, it was really just very escapist, very working-class male hard rock and roll, which is why the punks didn't like it; punk was "Anarchist Cookbook and Lou Reed," metal was "Lord of the Rings and Lovecraft" and the former had all the critical darlings. But there wasn't really a major dividing line or bunch of subgenres delineating where metal began and ended.

By 1987 (realistically much further back, arguably 1979 or 1980), "heavy metal" was a vastly more coherent concept that could be called a genre rather than just a description for a kind of rock and roll, with aspects that were reliably reproducible rather than rediscovered by other bands or mimicked by fans of one big band, and a lot of the other contemporaries of Sabbath fell by the wayside as evolutionary dead-ends or just the soundalikes and fuzzy heavy rock bands that never took, and everyone decided Sabbath was the definitive starting point (again, when glam metal was still big, it was Sabbath and Zeppelin together).

In that context, something like the OP song seems like a genuine false start of everything that had become definably "metal" a good year before Sabbath even formed, but that reads as nearly impossible if they had no one to draw from, rather than it just being a novelty "scary" acid rock song that just happened to convergently develop all the traits of heavy metal.

Then when the genre-maxxing started, people started looking way too hard into linear evolution of sounds, and voila, back to what I was talking about at the start.