It's because the ability is not active yet (so !IsActive returns true), and m_ItemSetManager.GetActiveItemSetIndex(m_ItemSetGroupIndex) is returning the same number as the provided itemSetIndex (which, I should think it should be because that's the item set [the currently active item set] I want to unequip).
I went through my code again, and I think it was because the default item set had the item equipped, so on EquipUnequip.cs Line #1163 m_ItemSetManager.GetDefaultItemSetIndex(m_ItemSetGroupIndex) was spitting out the same active index number resulting in the above listed conditions. I had not noticed before that it was getting the default item set index.
I should think that listing a blank item set where the item starts unequipped will cause the code to work as I want. But the problem comes from the fact I want the character to start out with the consumable equipped and ready to use by default at the start of the game.
I've actually already found a work around for my specific use-case that fits more in line with the mechanics and controls for my game, so I'll make this as "solved", but having the ability not start when I want the active/default item set to be unequipped seems like an oversight to me.