In general, when you gain Rune Magic from a hero cult, that magic comes from the god behind the hero. Where the hero is going to be apotheosised as a god, then you can once that hero is no longer active in the mundane world.
The magic comes from the Gods World. But the cult is to the god who represents that archetypal event or experience.
Although individual heroes can gain such magic through their direct experiences in the Gods World, for normal mortals that requires participation in cult that can provide a less dangerous (and more reliable) approach.
Although some claim they do just what you described through worship of Arachne Solara. However most claim that the magic the Cosmic Spider provides is indirect only.