The historian: "Enacted" during the period in question (the third Dynasty)
The critic: "remains on the books" (i.e. has existed from before; hence was enacted before the third Dynasty).
The historian argues that the law "was enacted during the third Dynasty," hence the suggestion of trade during this period. Why would the third dynasty "enact," meaning, legislate and pass a law in a period, if the times in the period don't ask for it! This is indirect evidence.
However, the critic misses the fact that the law, per the historian, was enacted in the very time period in which the historian suggests there was trade. Instead, the critic by way of a temporal analogy makes it sound as if the law had been in existence from before the third dynasty period - "today many laws remain on the books..."