I'm not so sure the bone was retconned in.
The pirates who peeked at it like 50 chapters ago called it gross. We also don't know exactly what kind of person Ken used to be but the guy was definitely well educated and knew quite a lot about this era of history as well as the various historical development of dishes all around the world.
His father being an archaeologist who also made sure to pass on some outdoorsy lessons personally I'd hazard a strong guess that Ken had to have been something of a history buff himself. To go further, I propose he was a fan of the Sengoku era figures and Ken's dad probably figured he'd get a kick out of what might very well be Nobunaga's bone relic.