Has there actually been anyone that called 764/com and it's birth-defected children as 'far-right extremist'? I can't see it getting that label other than a desperate reason/scapegoat of why the sick fucks do what they do.
In beginning, when the picture was a bit more muddy, that's what I originally thought it was, because of the whole "Oh, Atomwaffen is involved here."
As time went on and I understood more, and likewise other sources understood, it's less about the right wing "Hail Hitler" and more of a "Haaaaaaaaaaaaaaaaaaail Hitleeeeeeeeeer XDDDDDDDDDDD" sort" who don't actually practice the idealogy and just like the aesthetic for particularly edgy things because somehow it gives more rep; as seen in your hyperborea anime girl/AI video nonsense that is utilized as part of its propaganda wing.. That being said, some of the circles such as NLM/Saubern/MKU(Y) can be considered more traditional alt right terrorism, but frankly only really because of the aesthetics rather than the idealogy.
The idealogy of said groups is chaos, acceleration, and nihilism; the second term, acceleration or accelerationism is something that is typically seen in law enforcement as a relatively alt right thing, because frankly, that's where you see most accelerationism occur. The people who sabotage power systems, the "build our armada and militia" types, your uncle Ted worshippers, the ones that go after government employees and buildings and such; are typically right leaning, thus in the mind of the investigator, accelerationalism = alt-right. Which ultimately, I think, is incorrect since chaos, accelerationism and nihilism, even with noctillianism (which is a term for satanic nazism) is something that is it's own idealogue seperated from politics or others. These people don't care about bringing about a 4th Reich, revealing the Jew, or ensuring a future for the white race; these people want to destroy society because they hate it as much as they hate themselves.
Long story short; it's mostly perceived that way because that was the initial take, and it's going to remain that way in the public reporting eye, because it's easier to explain it as such to a wider audience. "It's an idealogue of political based extremism"
Trying to explain to a wider audience of "It's literal the concept of evil being made manifest through the acts of Man, that masquerades as whatever it wants to" is a bit harder.
Remember Die Hard when the CEO asked "What kind of terrorists are you?" and Hans Gruber just smugly smiles? That's what we are dealing with here.