We judge metal by the standard of HLTO; it is probably the greatest achievement in metal. The works of Schubert cs. are it's equivalent to classical/music/art.
judge metal by that standard. If that's what you meant, then I sense this is a soft way of saying that Burzum is the bench mark of all metal. The problem with a claim like that, and the others you've made, is not that it's controversial, it's that you don't substantiate it. To go on and say that Schubert is its equivalent in classical is, I think, demonstrably preposterous.
Compositionally, it can be demonstrated that their techniques and goals differed considerably, and that the resultant product did also. Consider for example Schubert's interest in relatively local events, small themes and catchy tunes, repeating them largely whole sale, even in works typically associated with variation technique (sonata form, ex. symphony #2 1st mvt.) This is in contrast to Burzum's focus on relatively global aspects of compositional design, as for example in the album you mentioned Hvis Lyset Tar Oss, where the number of riffs per song exceed the average for a black metal band by far. While you may point to "Tomhet" as an example of Varg's emphasis on local repetition, its lineage is from 70's and 80's synth bands, who in turn were influenced by the early post-war minimalists like Morton Feldman and Steve Reich, who were themselves reacting against
models of composition that Schubert typifies.
Ideologically and personally, they were also nearly total opposites. Schubert was a savvy cosmopolitan city dweller, who held swanky cabaret parties at his house for the bohemian strata of Vienna. He was also likely a homosexual, and died of syphilis. Varg Vikernes is an unrepentant radical traditionalist, references his own introversion and even anti-socialism in interviews, and lives on a farm in the countryside.
Or did you mean "equivalent in quality", and not spirit? You've yet to show how either match up to any standard model of "quality" as such, yet to define that model, and yet to show how their relative levels of quality are not only equivalent, but in fact the highest(!).
I invite you to. Otherwise, how can we even react to statements like these?