As I've already mentioned in the comments - I'm not so sure if War Backpack is wearable. Fansite items have to be useless, just deco - shields have def. 0 etc. It means that War Backpack can not be a container!
- Where is info about no slot in War Backpack?
You don't need any excact info. All you have to do is to click "look" on this item.

https://www.tibiaroyal.com/pl/guide/item/1226/war-backpack/
As you can see on screenshots here, War Backpack is writable! Moreover it has no info about slots - "(vol.20)" which you can find on any other backpack-container.
To sum up - maybe it is possible to wear it in the container slot (we don't know it), but I'm sure that we can not use it as a container like any other backpack.