I think that’s exactly the right distinction, Jonah. In fact, I think it’s exactly the distinction between the Spartan Rhetra and the Athenian Constitution.

I’d dispute with your correspondent, though, that Magna Carta was baronial blackmail. John had overturned law and tradition and the barons were trying to restore it (John’s resistance even after the signing at Runnymede led to them allying with the French). Take a look at the Charter of Henry I Beauclerc, which refers back to the laws of Edward the Elder. There’s a tradition there that goes back to the earliest times.