Spitball Theory: Calamity of Lightning. Solution Nine is the last surviving remnant of the civilization that rose after the first calamity and that the First Astral Era did reach a little beyond "to forge stone tools and established settlements and towns, which grew into kingdoms that soon went to war, each seeking to expand their domain." Just a tad. And to add to the Shinra vibes, "To forge their tools and weapons, mountains were gutted and skies blackened." So, this society was older than Allag but younger than immediate Pre-Sundering. Enough so that the proto-script survived unaltered. In Tural the memory of this civilization survived as folktales said as Allag did back east, with the same recentish rediscovery by North Tural's version of Saint Coinach, hence Heritage Found. Nothing wackier than that.