The word “Presbyterian” comes from a Greek root meaning “elder.” The Presbyterian church uses the name because their structure has the church directed by elders (lay spiritual leaders who have been involved for some time and hence tend not to be young); the cannibals are using it as a more generic reference to old folks.
Ah, I see. It’s a pun based on Greek. The “elders” in my Presbyterian church (when I was growing up) were of all ages, including some not quite 20. I thought that funny at the time.