Of course you're right. But the rules are not entirely unknown. For example, archeologists found the pieces. So, they know the dice are involved. And they likel

Of course you're right. But the rules are not entirely unknown. For example, archeologists found the pieces. So, they know the dice are involved. And they likely correspond to rules for other similar games (roll and move, like backgammon or Ur). So, it's not entirely black and white. Historians are pretty sure about some of the rules, but it's possible there were other rules we don't know. The rules played today are a balance between historical accuracy, and just trying the different theories and playing the one that seems the most fun. That might seem like just making things up, but that's what the quote is referring to. It's hard to say what set of rules was the most common, but there is some reason to believe that the current set of rules is as least similar, and might have been played due to the way that these games didn't have 1 concrete set of rules. "House rules" meant that there was often many different sets of rules in place. Think how checkers has sets of rules that allow for non-forced jumps, flying kings, etc. Maybe your can't tell whether your set of rules was the most common set, but if your rules are based on evidence, it's likely that it was played similar to that somewhere.