the general attitude in the oto is that it is immaterial as to whether or not the aiwass myth was founded in some real happening. the book of the law and the other inspired writings of aleister crowley stand on their own and are so dang sweet that they might as well have come from space or gods or whatever.
this is curious because to take the I° in oto you have to "accept The Book of the Law," and most people are like "okay, i accept it..." But Crowley comments somewhere that this implies accepting that it comes from a praeterhuman source. why would crowley demand this? the reason is more obvious in teh first degree. suffice to say that this idea of the Secret Chiefs is integral to Thelema. Most people mock this and say "Crowley needs you to believe that his authority comes from an inaccessible higher power." But I think the answer is actually quite simpler.
To accept the Secret Chiefs (and thereby the possibility of transmitted gnosis or doctrines or whatever) is to accept the core principle of Thelema which is "knowledge and conversation with the holy guardian angel," which is, Crowley later wrote, communicating with a dead magician/mystic, aka someone who has (to quote the Stele of Revealing) walked after death or otherwise managed to extend their consciousness, identity and Will beyond the limitations their previously animated corpse allowed.
so I think it's less about saying that The Book of the Law comes from god's butthole and more about saying that one accepts the possibility that one can extend one's life beyond one's death and one can communicate with these spirits or "angels."
Do I believe the narrative? For me, as for most magicians who are practiced in evocation and astral work, the possibility of discarnate independent intelligences is not silly like it is to most people. I have had many visions (some contrived, some somewhat prophetic, if only on a personal scale) where i interacted with a spirit that knew more than me or otherwise told me something i didn't know. So frankly, the idea that Crowley could have Aiwass (the true god of the Yezidis, some dead Levantine guy) visit him and dictate a book isn't that implausible.
Do I need to believe in the narrative? Not really. In Liber O Crowley writes:
1. This book is very easy to misunderstand; readers are asked to use the most minute critical care in the study of it, even as we have done in its preparation.
2. In this book it is spoken of the Sephiroth and the Paths; of Spirits and Conjurations; of Gods, Spheres, Planes, and many other things which may or may not exist.
It is immaterial whether these exist or not. By doing certain things certain results will follow; students are most earnestly warned against attributing objective reality or philosophic validity to any of them.