Just purchased an Edict from the Rat Market and the dialogue made me pause, then laugh.
“Proper translation will take some time; but the few word stems you recognise suggest that it concerns the repayment of debts.”
Given that the First City is Mesopotamian, isn’t it possible that the Edict of the First City is…the complaint tablet against Ea-Nasir?
As a historian: recording transactions and debts is a large part of why systems of writing were developed as part of the shift to agrarian societies with subordinate farm workers, so it’s certainly not surprising that any given example of an early text would be a financial record.
As a humorist: oh, that absolutely says ‘you owe me big for that crappy copper’.
I think the in-universe explanation likely has something to do with the Creditor storyline, as the item was added in the course of the Railway updates.
On the meta level, you may well be right that the writers intended that as a little wink.