Yes, Stuff is Material, but in modding terms, a Stuff is a material in a recipe that can be changed to make variants of an item. The material filter looks for this Stuff.
Think about it this way, have you seen a revolver or a bow that are have the material they are made of in their name.
I would be pretty sure the answer would be no unless you have some mods that change this, because these are items that don't have Stuff, they are only displayed as 'Revolver', in the sculpture example 'gold small sculpture' has a Stuff 'gold' shown to the player in the name of the item. you can tell if any item has a stuff by keeping this in mind.
Because the bow has no Stuff Material, the Material filter doesn't work in the context of what you're trying to do. The Bow is classed as 'No Material'
The rule you are making is a mix of weapons with and without stuff materials (bow being without a stuff), but because the Material filter is looking for wood on items that have no stuff, it fails on those weapons.
That is why the Material filter doesn't work with what you're trying to do. The Material filter isn't suitable for that unfortunately.
the example in my previous post shows separating variants of the same item type (small sculpture in this case), where the material filter can be used.
I'm trying to keep the technicalities of stuff and how they relate to recipes to a minimum, sorry if it isn't terribly understandable