Correct me if I'm wrong here, but for all we know there's no actual proof that the notes he found were in fact left by a past self at all, the MC just believes it to be so. It could very well just have been two random people that believed in reincarnation, and one of them just found the other's notes.
Aelxer t1_j622nrp wrote
Reply to comment by FarFetchedFiction in [WP] An elderly man, who believed in reincarnation, decides to leave a series of notes in the pages of obscure books at a historical library. As he starts hiding the messages for his next life, he gets a shock after finding a note from his past self... by metamazenft
Correct me if I'm wrong here, but for all we know there's no actual proof that the notes he found were in fact left by a past self at all, the MC just believes it to be so. It could very well just have been two random people that believed in reincarnation, and one of them just found the other's notes.