The narrator explains that fate has always existed in the divine realm. The Three Sisters of Fate in Greek mythology are said to decide a person's destiny within three nights of birth. The first sister, Clotho, is a young maiden who spins out the "thread" of life. Her middle-aged sister, Lachesis, decides how long the life is, and what will happen during it. Then the crone, Atropos, determines how and when the person will die.
The discussion of the Greek interpretation of fate and destiny illustrates clearly how the idea of fate manages to still infiltrate people's lives, even if one doesn't actually understand how the three sisters necessarily functioned.
Though the three sisters no longer figure into people's beliefs, the idea of fate persists. The narrator wonders if it makes tragedy more bearable. Patricia believes that there's comfort in knowing that things are out of her hands, but Natasha and Dae Hyun believe in determinism: one action leads to another. Daniel lives somewhere in between. The narrator insists, however, that once Natasha and Daniel met, whether it was fate or not, their love was inevitable.
Again, Yoon's choice to say outright clearly tells the reader that within the world of the novel, fate and destiny exist and are somewhere in between true determinism and true destiny. Essentially, it's possible to pick out the cause and effect retroactively, but destiny still caused it.