Is the Order of Nine Angles a myth?
No.
It was initially founded by Myatt in the early 1970s as a means of recruiting aspiring British occultists into Colin Jordan's National Socialist Movement. In that sense, the ONA has been a supremely effective magical working; under the symbols of Traditional Satanism it has introduced tens of thousands to a remanifested Völkische Bewegung who would have otherwise never encountered such ideas. It's not been far away from the recent political upswell of those ideas either, especially in the early stages of 4Chan.
In the 70s and 80s they remained a small group in Shropshire and Yorkshire but there were always more than Myatt alone. In the 90s, they were closely aligned with Stephen Cox's homoerotic-pagan group, Fraternitas Loki and many members overlapped. Myatt withdrew from all ONA connected work in 1999 when he became a "person of ongoing interest" to the British government, who suspected him of being the mastermind behind David Copeland's London nail-bombing campaign. The police could never find the smoking gun to charge Myatt which, given the content of the early ONA MS, is why he cannot admit to being their author; they'd then have their "incitement to murder" evidence. At around this time, the ONA original MS were published online at camlad9.tripod. Myatt is sincere in his Numinous Way work. He renounces the former movements with which he was involved.
I came along in the middle of the 00s as a young initiate of Fraternitas Loki. The ONA were still a mostly British based group at that point, led by, and this is not disclosing anything that's not already out there, a musician and artist called Richard Moult/Christos Beest. As is often the case in these things, the reality wasn't quite Nietzschean self-mastery but I learnt a great deal and it prepared me for my later encounter with something on a different wavelength. The ruling principle of "My Life will disclose where I am and only cowards avoid that mirror" has set me in good stead even if it makes failures all the more keenly felt.
A few years after that encounter, the internet discovered the ONA MS and they became a mandate for every non-entity to indulge in every mindless, destructive impulse they could muster (and a handful of nobles to rise). Often these were flagrantly in contradiction to the original spirit of self-overcoming that the ONA espoused and included child abuse and revelling in chaos rather than using it (as a general rule, if they call themselves O9A rather than ONA, they're not associated with the original -
, perhaps to discredit a method of transformation). Today, the documents are public domain. Anyone can use them and add to them. Little of what has been added is of any value.