This may be just a bunch of weapons that the devs didn't include in the finished game, but kept the models in the game to use for other things then weapons. Because if you look at the inventory icon images, there are no images of the weapons. Probably because since the devs were not going to use the models as Player Weapons, they didn't need to include any Inventory Image Icons. Makes sense.
Or am I completely wrong? lol. After all, that is just my best guess.
I say that, because I don't think you can actually add "New" weapons, but only "Replace" weapons that are already in the game. But again, I could be wrong, since I never actually modded the game files in BOTW. I have only edited my Saved Game with a Trainer and Online Save Editor.