Pyramidiocy in the "Ancient Texts"

One of the incessant refrains of alternative theorists of all stripes--from Atlantis believers to ancient astronaut proponents--is that "ancient texts" are the key to understanding prehistory. So what do we do when the ancient texts are demonstrably false? Here is one describing the pyramids of Giza from c. 825 CE, written by the Irish monk Fidelis, possibly a pseudonym of Dicuil, who wrote the Liber de Mensua Orbis Terrae (c. 825 CE): “After sailing on the Nile for a long time, they saw, like mountains, the seven storehouses […] which Holy Joseph had built, four in one place and three in another” (3.2; my translation). Well, after the pyramids were opened, we learned that they were not, in fact, hollow storehouses for Joseph's grain. It's also fairly doubtful that Benjamin of Tudela (1130-1173 CE) was correct when he wrote that the pyramids "are constructed by witchcraft" (Travels, trans. A. Archer).

So, why then should we do as the alternative theorists would have us and believe al-Firozabadi (15th century CE) when he writes that the pyramids "are supposed to be either two antient buildings erected in Egypt by Edris, to preserve the arts and sciences, and other knowledge, during the deluge; or the buildings of Sinan Ben el Moshalshal; (or the buildings of the antient antediluvians), erected in consequence of the stars foretelling the deluge" (trans. Alois Sprenger in Vyse, Operations)? Just because the al-Firozabadi can be twisted into to support for Atlantis or aliens while Fidelis is provably wrong doesn't give us much reason to trust a late medieval source over an early medieval one--or, for that matter, any medieval source, written 4,000 years after the Pyramid Age.

The wisest words about all of these myths and stories were written by Diodorus Siculus 1500 years before al-Firozabadi. Diodorus, with a bit of numeric exaggeration, wrote that "Some of the Egyptians tell wonderful things, and invent strange fables concerning these works [...] But this is not the truth of the thing; but the great multitude of hands that raised the mounts, the same carried back the earth to the place whence they dug it; for they say, there were three hundred and sixty thousand men employed in this work, and the whole was scarce completed in twenty years time" (Library 1.36, trans. G. Booth). But of course, no alternative theorist believes Diodorus. That's just ridiculous.