Although I'm not too familiar with Hail Spirit Noir (iirc I hated a song of theirs posted in one of the mixtape games), at least 70s prog is often rooted in guitar-based, heavier rock music. King Crimson, Genesis, Rush, and others had their moments of surprising proto-metallic heaviness. That's quite different from DSBM bands that just twinkle on a few chords for an hour straight, or post-rock wannabes with harsh vocals and blasting, or bands that mix three parts of MIDI folk melodies with one part metal and call it "folk/black" or "symphonic/black". Yet when a band with riffs entirely derived from thrash or death metal decides to add screamy vox or metalcore breakdowns, they're immediately booted from the cool kids' table. It's a double-standard and black metal is full of treasonous elements that are only just starting to be rejected because of its growing popularity (e.g. Sunbather), but underground stuff with literally no metal riffs gets a pass because of aesthetic choices.