Not so. The Confederate fire did considerable damage to Fort Sumter. They not only used cannon, but also mortars that lobbed shells from above into the Fort.
"It became apparent that the Confederate batteries had begun firing 'hot shot,' cannonballs heated in furnaces. One or two balls came to rest inside the fort, where one of them set a man's bed on fire.
"At about nine o'clock, a shell from a mortar burst through the roof of the officers quarters. Heavy smoke rose from within. The location of the fire was to exposed to allow men to effectively fight it, Foster realized. He alerted Anderson that if the fire continued to burn out of control, it could detonate the fort's cache of gunpowder, the thirty thousand pounds stored in barrels in Sumter's main magazine." (The Demon Of Unrest, Erik Larson, Crown Publishing Group, 2024, p. 440)
"...about nine a.m. a loud cheer rose from the direction of the beach....the sound was deafening. It goes on from hill to hill till it reaches the farthest end of the Island. Fort Sumter, they saw, was on fire." (Larson, p. 441)
"The Confederates fired at a faster rate, with the obvious intention of worsening the blaze. At this they succeeded....By eleven A.M. about one-fifth of the fort was on fire, by Captain Doubleday's estimate." (Larson p. 442)
"The scene at this time ws really terrific, Doubleday wrote. The roaring and crackling of the flames, the dense masses of fwhirling smoke, the burstin of the enemy's shells, and our own which were exploding in the burning rooms, the crashing of the shot, and the sound of masonry falling in every direction, made the fort a pandemonium." (Larson, p. 443)
Quantrill