Presumably either time travel or a certain raffish inaccuracy is involved. The Hitchhiker's Guide is not known for its strict attention to factual detail; other Guide entries we have seen claim to have been written by the 17th-century English diarist Samuel Pepys and a WWII Soviet naval cruiser. :)

--G.
That is not to say that none of those citations is true, mind you. Only that the book has a certain reputation. :)
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