An oath not to make weapons requires the person to know what a weapon is, and to think of whatever they're making as a weapon (or not). A crazy person who doesn't remember which weaves are weapons and which aren't, or who doesn't think of their weave as a weapon, would not be restricted by the oath, just like Aes Sedai can still tell a lie as long as in their minds they think it's true.

tl;dr, an oath on the rod requires a certain degree of cognizance to make it work, since the person's perceptions of their oath and their actions are what binds them.