That is correct. The reason you collect 45 fragments is this: Lore wise your guardian stumbles upon an early calcified fragment and reports it to Eris. She is intrigued by this because it is knowledge of the Hive that even Toland was not aware of. She requests that you bring her more and she gradually pieces together the purpose of the book. Seeing an opportunity for the ultimate retribution and knowing your abilities as a guardian, she wants you to end oryx and collect the pieces necessary to craft the weapon in the process. Finally, to cement her revenge on the Hive after killing Oryx, she constructs the Touch of Malice for you. Now knowing that there are still the other two sisters' broods remaining there is no sweeter justice for Eris than knowing they could at some point be ended by a weapon crafted from the essence of their own sister, whose son stole those who she cared for and her ghost (her connection to the light). Committing a retaliative genocide on the Hive using their own power is probably a fantasy of hers.