In the 22-years-ago flashback, the legendary shipwright Tom is sentenced to Enies Lobby execution for having built the Oro Jackson — the pirate ship of Pirate King Gold Roger (pp. 19–20). A historical World-Government judicial-island death-march that directly prefigures the present-day arc’s climax: the Straw Hats’ raid on Enies Lobby and CP9’s pursuit of Nico Robin and the Pluton blueprint. The flashback also seeds why CP9 hunt Franky — he is the surviving apprentice and blueprint heir.
The reveal welds the Water Seven Pluton-blueprint custodial lineage (Tom → Iceberg → Franky) to the Roger era and the Void-Century weapon mystery: the master who built the King of the Pirates’ ship trained the two men who inherit the plans for “the world’s most terrible battleship.”
Why it’s a seed (not yet resolved)
The structural rhyme (Tom’s Enies Lobby trial → the arc’s Enies Lobby raid) is laid down explicitly, but the full payoff — the raid itself and the fate of the blueprints / Robin — is forward of Ch. 353. Kept status: pending.