It was my mistake not to explain this correctly in a devlog or documentation; the Weapon Unlock option was a later addition, and I failed to explain its functionality.
It doesn't unlock the weapon itself; my plugin doesn't make any inventory changes. This function unlocks the weapon type, and this ID is the weapon type ID in the RPG Maker database.
With this node, you can allow a mage to use a broadsword or a martial artist to use a bow. It basically enables a weapon type that wasn't natively available to your character's class, making it available.