Although he said so, the real reason Aiden didn't act was that it wasn't the right time to strike.
Melissa had once provided him with a crucial piece of intelligence: in general, a person could only create three rules in one day using the Wordless Code.
To successfully break out of prison, Navi Derrick would likely need to write down two rules on the Wordless Code: "Prohibit attacking and causing harm" and "Prohibit restricting personal freedom." With these rules in place, they could easily leave the prison.
But if he wanted to escape successfully back to sea from the autonomous region, he might be persistently pursued. If his escape route was anticipated ahead of time, the autonomous region and the Empire could still make it hard for the pirates to leave land by destroying roads, bridges, and boats—as long as these actions occurred outside the influence of the Wordless Code, they wouldn't be considered "restricting personal freedom."
