I headcanon that while Spinner was writing his book about the League and Shigaraki, he asked to meet Izuku one more time so he could hear again about Shigaraki's final battle.
Despite Spinner's deep resentment toward heroes and hero society, he and Izuku found an unexpected connection in their shared grief. Both had lost Shigaraki, albeit in very different ways.
Spinner mourned his friend, the person who gave him a place to belong, while Izuku mourned the boy hidden beneath all that hatred, the person he believed could have been saved.
As they talked, they bonded over the "what could have been."
Spinner spoke about the future Shigaraki was never allowed to have, and Izuku spoke about the life Tenko Shimura never got the chance to live.
Neither could fully understand the other's perspective, but they understood the pain. In the end, their conversations became less about heroes and villains and more about remembering the person they had both failed to save.