"Resisted" does not necessarily mean "fired upon." Captain Mercer could have interpreted it as anything which interfered with his effort to put supplies or men into the fort.
Just blocking the channel could have been seen as "resisting."
Mercer’s actions speak for themselves. His orders were to use force only if the resupply effort was resisted. He did not allow his forces to fire on Confederate forces even though he was fully aware that they were firing on Fort Sumter. Not one person in his military chain of command would have found fault with his actions if he opened fire on the batteries that were firing on Fort Sumter. He interpreted his orders very strictly. He would not have opened fire unless the resupply mission was resisted by force. The fact that the Fort was being fire on had no bearing on his orders. Even the Confederate battery commanders were amazed that the Union war ships did not make a move to aid Fort Sumter. Why, because their orders were to only use force if the resupply mission was resisted.
They were not sent down there to open fire on the Confederate forces, They didn’t have “ATTACK” order from the President.