Currently if an item is defined in config.cpp with an inventorySlot as string, it will not allow to be overriden as an Array. It has been tested in multiple ways.
Could you please update config variable inventorySlot to be ONLY array? For example Hatchet has inventorySlot="" in the config which disables the possibility of making it into an array through modding.
From: inventorySlot="CableReel"; To: inventorySlot[]= { "CableReel" };