Fair enough, but I'm thinking more of something like how they got George R. R. Martin to write just the foundational mythology of the setting. Also, I think the "item description lore" of From Soft games and Type Moon's "material book lore" type of approach are close enough.
Yeah, that's why I specify the Lostbelts. Thinking about it, the concept of the Fantasy Trees and the Erdtree are kinda similar.