Lovecraft drew inspiration from Theosophy’s (fake) Book of Dzyan in developing the Mythos’ own pre-human or occult texts. While Blavatsky attributed Dzyan to Tibetan priests, Lovecraft reconfigured Dzyan to “antedate the earth” in “The Diary of Alonzo Typer,” the same story in which he made Theosophy’s spirit Venusians into actual aliens who flew across the solar system in space ships (Theosophy's version merely projected their minds, something Lovecraft used in the Shadow Out of Time). Interestingly, in Gods from Outer Space, Erich von Daniken also said Dzyan was “older than the earth” (p. 137)—the same words as Lovecraft, but not Blavatsky.
Jacques Bergier and Louis Pauwels, authors of Morning of the Magicians, followed Lovecraft’s interpretation of the Lords of Venus as flesh-and-blood aliens on spaceships and the Theosophical notion of spirit beings from other planes of existence. Both authors were big fans of Lovecraft, and Bergier claimed to have corresponded with the Providence author (though no letters survive). Bergier and Pauwels take the Book of Dzyan seriously as a real piece of ancient myth. On page 121 of the first American edition, the authors write that:
while the shelves of our specialized libraries are loaded with a whole literature labeled “legend”, no one has ever thought for a moment that this label may conceal picturesquely presented accounts of events that actually happened.
And yet, with our knowledge of modern science and techniques, we ought to examine this literature with an un-prejudiced eye.
The book of Dzyan speaks of “superior beings of dazzling aspect” who abandoned the Earth, depriving the impure human race of its knowledge, and effacing by disintegration all traces of their passage. They departed in flying chariots propelled by light, to rejoin their land “of iron and metal.”
Immediately thereafter, the authors go on to claim (falsely) that ancient texts tell of nuclear war.
As best I can tell, the oldest claim that Dzyan is extraterrestrial comes from the 1916 book Christian Wisdom, in which Theosophy is reconciled with Christianity. Franklin Ellsworth Parker writes:
The Aryan civilization is one of the oldest civilized nations of earth and the book of Vedas next to the Dzyan the oldest work. The book of Dzyan was written by moon souls 24,000 years ago and 2000 years later Brahmin, a Jupiter spirit father, wrote the book of Vedas from the book of Dzyan, but mutilated on questionable lines to suit the convenience of his own selfish personality and obscure the universal soul principle of Divinity. […] Ancient philosophy was the religion of eternal truth and the greatest teachers lived in the Himalayas. (pp. 174-175)
However, these “souls” were not exactly aliens but rather the spirits of beings that lived on the parallel universe (“etheric plane”) version of Venus and Jupiter before projecting to our reality to guide human evolution; Parker also implies the book was written on earth. Parker, a sort of Christian Charles Fort, manages on a single page (p. 172) to claim Dzyan was written in the language of Jupiter as well as that of the Moon (which is Sanskrit, in case you visit).