Patroclus is using the same coping mechanism he used when he took the oath and had sex with Deidameia: he pretends the things Achilles tells him are fiction. In doing this, he’s distinguishing between the Achilles who kills and the Achilles he loves, pretending that the killer is a character in a story and the Achilles he knows is the real one. That’s both true and untrue. Achilles
is literally a character in this novel, but murder is still a part of how Madeline Miller presents him. This implies that Patroclus shouldn’t bother to distinguish between the two different versions of Achilles and should instead accept Achilles as he is. After all, Achilles’s complications are how he’ll be remembered later—literally, because readers see the full picture of who Achilles is in this novel.