Cicero was talking about yet another thing - reason-based law. Natural law like physics is amoral - a falling rock doesn't care if it kills a good man or a bad man. Statute law is inconsistent - it's supposed to punish only bad behaviour but it doesn't always work out that way, and it is not necessarily the same in Athens and Rome. Cicero apparently thought there was a third kind of law based on reason, that was universal to all humankind, and thus would be the same in Athens and Rome, and is "known to all men". However that is a questionable assertion, given that two people can both claim to be reasonable, yet have very different opinions, so whatever the right answer is, it isn't "known to all men". Plus Cicero was well aware the law was in fact, different in Athens and Rome. And he wrote the book as a dialogue with people arguing different points of view, so presumably the piece you quoted was one character's point of view, to be challenged by another character who disagreed.

However there is no doubt that the Latin motto you quoted is intended to apply to statute law.