Bernard,
Not to be a troll or argumentative, but surely you would admit that it also stretches credulity to believe that a society whose scienctific/mathetmatical capabilites exclude knowledge of such concepts as the wheel, pi, precession, and heliocentrism would have the engineering and organizational skill to begin and conclude a decades long project to construct a megalithic structure so precisely aligned to the cardinal points that it continues to awe modern scientists (to wit: Prof. Owen Gingerich, Harvard University).
Even if you refuse to admit that knowledge of such engineering and organziational skill stretches credulity, surely you would acknowledge the prescience of those mathematically challenged ancients to know that the life expectancy of the Pharaoh would be within the time required to construct this structure.
Barry