This sentence reminds me of the stock phrases we use to introduce or end fairy tales. The “thus it was” could function in a similar way, as a closing formula to a certain type of narrative shared by a specific (the Lakota) culture. The sentence serves as some kind of concluding wrap-up comment on the story told and it emphasizes the relevance of both the “Beautiful Woman” and her precious gift. It marks the preceding narrative as a story about the origin of an important cultural ritual and object and therefore makes it very suitable for ritualistic use and retelling.