That's the great thing about playing in the Shadowrun Universe...everything is available to the GM and I mean EVERYTHING!
Was Stephen King actually an early arrival shedim with a penchant for the written word who never had to sleep and could write 24/7 365...sure...or he was just a prolific, human, writer.
Lovecraft gained insight of the horrors...sure...or he was just a writer in the early 20th century?
Dr. John Dee actually summoned spirits and recorded the alphabet of spirits during a mana spike in the 16th century...sure...or he was just a very interesting, but mundane, member of Queen Elizabeth's court?
Was El Dorado actually an materialized Alchera that returned to the astral before the Spanish could find it...sure...or it was just a myth.
12th and 13th century Jews in Spain actually formulated a path of initiation via the Kabbalah and the Tree of Life diagram (one of my favorite characters I played was a Jewish Troll magician who used Kabbalah and I assigned the different metamagic techniques to the Tree of Life who could only get the techniques by following a path from one to the other, couldn't learn a technique if he didn't get the others lower on the tree first)...sure...or where they just eccentric students of the Torah?
Does the Book of Enoch (1 Enoch) actually describe an Astral journey to the metaplanes, near the lowest point of the 5th age's cycle at ~300 BC...sure...or is it just one writer's attempt to make sense of Sons of God/Daughters of Men section in Genesis?
I could go on, but Tolkien learning bits and pieces and putting it together into a largely fictional (but with hints of the "truth") story that included the a primer on the elven language, or an elven language...sure...or he was just a really thorough and meticulous writer that happened to survive one of the deadliest battles of WWI.