I think someone didn't read the chapters from the beginning properly this is a side series of when baji was dead in the Valhalla arc when chifuyu was visiting baji's grave he met his mother who gave him a letter that baji wrote. That's why it's called a Letter from Baji's and the story is before when chifuyu became the 1st division vice captain and joined toman so this is not related to ch 277 of the main series it's a side novel. Idiot.