It doesn't need aliens. The people would have to encounter such things as the ruins of Jericho (destroyed at the beginning of the new kingdom period), or later cities burned down during the late bronze age collapse. Either could easily represent an extent of destruction incomprehensible to unsophisticated herdsmen. Later it was Greece or even Rome itself, before the area became a part of the empire. It's pretty clear that angelos was something like a courier or mailman, for example, and only later it acquired the mystical meaning.