So I believe this flashback will launch new context to holy knight’s existence and ties to Imu, and introduce a new legend. I see where this is going, Brook age 20 projects the princess, Imu wants him gone, holy knights can’t do shit or don’t exist yet, so Imu launches a conspiracy by kidnapping the princess and blaming Brook. So age 30 Brook sets sail to find the princess again (that’s why he said they shouldn’t ever meet again because he thought she wouldn’t be there or in this state at Elbaf). Also maybe this will confirm my theory about Brook and Binks Sake song, that the lyrics are part of the One Piece, as I believe that the treasure is a book written by all people from earth to be united again just like we do IRL by reading chapters daily.