Assume not that it is true or false, assume that it’s a paradox (i.e. both true and false), and from that it follows that the king didn’t lie.
But, still, that’s not the only moral of the story. A moral of the story is also that we shouldn’t start by assuming some statements are either true or false, and then see what that implies about the referents, unless those statements are /entangled with their referents/. If statements aren’t entangled with their referents, then no logical reasoning from these statements can tell you anything about the referents.
There isn’t correlation between these inscriptions and implied contents (since he could have put the key and dagger in either box), but there /is/ correlation between {the inscriptions and contents} and the king’s honesty. The king didn’t lie and he wouldn’t have put inscriptions and contents into such an arrangement that would make it true that he lied. This puts a constraint on how he could arrange the inscriptions and contents.