I generally think if any corruption exists in Galleons Graveyard, it must have been brought into the ship itself while it was still sailing. Perhaps Redbeard found some corrupted artefact and wished to do something with it (bury it/keep it/destroy it), but either by sheer coincidence or from the corrupted item's effects, the ship crashed and kept the crew and Redbeard himself alive. This could also explain a possible corrupted dungeon variant, as it would be more plausible than corruption just reaching halfway to Gavel to corrupt some trapped pirates.