Learning what Olin really knows about the cultists is certainly an important endeavor for Aloy, but she also can't ignore what the cultists did to the Nora. So while you are working through A Seeker at the Gates, you will also have to complete two additional main quests involving the efforts of the Nora tribe to seek vengeance for their slain comrades. The dig site provides some clues on where the rest of the cultists went, and in Revenge of the Nora, you can clear them out of Nora lands in Horizon Zero Dawn Remastered.