The "necessities needed" was for the fort and was separated from what was listed just before it. It looks as if the arms were for the ship. Regardless of where what was going where, you can't show me the signed armistice so I'm doubting if it even existed. Looks like it was just a truce between the Buchanan Administration and the rebels. Plus any president has an obligation to give his troops the supplies they need to defend themselves so Buchanan was derelict in not supporting his troops if he did make a verbal agreement with the outlaws.
Does it not harken back the days when sailors lined the side of the ship and fired muskets at enemy ships? But then, the Union was not expecting an enemy fleet to engage.
After deep and prolonged thought, I have determined that the Union thought the ocean was seceding and they declared the Atlantic Ocean and inland waterways to be Enemy Belligerents and traitors and the arms were for the men on the ship to shoot the Atlantic Ocean until it submitted to the will of superior force.