koreanfestivals1-4, Exercise 2: Origins

Model answer:

Account 1: gabae. "According to popular belief", Chuseok originates from gabae, a month-long weaving contest between two teams under the third king of Silla; the team that wove more cloth won and was treated to a feast by the losers. The article adds that weaving, archery and martial arts contests "are believed" to have been part of the festivities.
Account 2: "Many scholars also believe Chuseok may originate from ancient shamanistic celebrations of the harvest moon", with new harvests offered to local deities and ancestors; the hedge is "may have originated".

They do not exclude each other because a contest and a harvest rite could belong to the same festival, and neither is tied in the article to a named source.
To choose between them, you would need a contemporary or near-contemporary document (not a later belief) that describes the festival, such as a dated chronicle entry, and a scholar's analysis of it.

Source: Wikipedia, Chuseok.