roger is a thing of the past, and oden is not even one of the original members, he was introduced around same time as bullet as roger pirate, and I don't think marines know OP location or else they would not just leave it out there for pirates to find it. this flashback should have been done early on in wano when they introduced the scabbards, not before the end. you do flashbacks when there is nothing going on in the present, not when its' full on strategy battle between beast pirates and SHs, when luffy's alliance is captured/lost, and full on war ebtween warlords and marines that represent the balance shift of the world.
You know Roger , oden have a huge role in it . I would say oden has got hype because he knows the secret of the world . The key man connecting the dots to raftel .
Marine doesnt know it but world government does thats the reason they dont want pirates go on tot race the mysterious island .
I am not sure its oda story he can start the oden storyline whenever he likes . Its just we can just criticize its normal since flashbacks ruins the the current events if it does not execute well .
Two thing i see why oda has introduced now :
1. He wants to show the world were scabbard were happiest nw they are in despair .
2. State of wano before and after kaido's invasion .
The flashback began with no factories and evrything was natural . Oda is kind of poetic in his way right now kinemon and other scabbard are looking hopeless since orochi destroyed their plan back then Oden would motivate them to not give up so its like paralleling the life with and without him . It shows oden had magical infulence for scabbards to follow him seeing hw optimist and positive vibes he had .