What they were ORDERED to do:
"Should the authorities at Charleston, however, refuse to permit or attempt to prevent the vessel or vessels having supplies on board from entering the harbor, or from peaceably proceeding to Fort Sumter, you will protect the transports or boats of the expedition in the object of their mission -disposing of your force in such manner as to open the way for their ingress and afford, so far as practicable, security to the men and boats, and repelling by force, if necessary, all obstructions towards provisioning the fort and re-enforcing it..."
IOW, IF ATTACKED by shore batteries, do whatever is needed to defend the resupply efforts. Which would then require using cannons on the Charleston forts. But the Union deliberately refrained from sending genuine warships, nor did they carry the thousands of troops needed to land there! The Pocahontas carried a whopping SIX guns. The Pawnee had 11 guns, but they were smaller. The Powhatan had 16 guns, mostly small. The Constitution, in contrast, carried over 50 guns. The sloops sent with the resupply mission were barely adequate to provide brief cover to the supply ships but ridiculous as an invading force!
The Confederates had already made it clear they would attack an unprotected ship. So the next resupply attempt was going to be made WITH SUPPORT. Just as we sometimes did when working resupply missions in Afghanistan. Supplies, plus force capable of screening the resupply effort if needed. If you know nothing of military tactics, please refrain from idle speculation.