Myth in Glorantha (and dare I say in our world itself) is not disguised history. Rather it is a way of understanding how the archetypes and symbols of meaning fit together into a greater whole. The Runes, the Twins, Ouroborous, the Rival Brothers, the Mother and Child, and other archetypes are combined together in stories and make patterns, some eternal, some subject to change or reinterpretation.
One can learn and study myths but as their sources arise beneath the rational the best is to experience them yourself and assembly these symbols personally.