Aside from being a really long song, I wouldn't consider it progressive rock or even an epic.
It's mostly just straight up acid rock. I personally find it to be a very boring and overly repetitive song, unlike the songs in this poll there's no real progression or complex structure or narrative that warrants it's extra length, it's just a 3 minute song stretched out to 17 minutes, probably because they were too high to remember how it ends.
I'm not an Iron Butterfly fan, they just sound like a generic garage rock band that gets way too excessive and spaced out, without having the chops for it or any real compositional direction or anything interesting to say with their instruments because it all sounds like rehashed ideas from Cream records. They were probably the most overindulgent, generic band of the psychedelic era.
The bassist gets points for being one of the founders of Captain Beyond though, now those guys were awesome.