Just to help clarify, but I'm 100% positive they mean that you can't item belt weapon swap something to an empty hand slot. For example:
1. Go into battle with no weapons equipped (have a weapon in item belt though)
2. Use swap weapon
3. You don't get to equip the weapon mid-battle.
EDIT: Added screenshots/GIFs.
1. Go into battle with no weapons equipped (have a weapon in item belt though)
2. Use swap weapon
3. You don't get to equip the weapon mid-battle.
EDIT: Added screenshots/GIFs.