Nothing fascinates alternative theorists more than Egypt, and the process of mummification has been attributed to a range of bizarre ideas, including but not limited to imitating cryogenic freezing and preserving DNA samples for the aliens to use in their experiments.
But the "ancient texts" make plain that the mummification issue was well-understood in Antiquity. Maurus Servius Honoratus, a fourth century grammarian, in his commentary on Vergil’s Aeneid at 3.67 writes:
“[T]he wise Egyptians took care to embalm their bodies, and deposit them in catacombs, in order that the soul might be preserved for a long time in connection with the body, and might not soon be alienated; while the Romans, with an opposite design, committed the remains of their dead to the funeral pile, intending that the vital spark might immediately be restored to the general element, or return to its pristine nature” (trans. James Crowles Prichard).
So, there you have it: The "ancient texts" so beloved of ancient astronaut theorists tell us that the standard explanation for mummies is in fact correct. Don't look for this ancient text on your favorite ancient astronaut/alternative history website anytime soon.