It’s not the rule, it’s a rather modern exception.

The word "rule" doesn't imply how old or recent it is (most rules aren't things like "rules of physics", they're things that governments, parents, bosses, teachers etc. make up quite regularly).

The rule (in this case, rule of law) that forbids slavery is pretty modern, surely that doesn't make it any less important?

Rule also implies universality, which is how I interpreted it here. I was saying this is a new, ‘limited’ belief.