"He abandoned his position...and withdrew, under cover of the night, to a safer position."
Methinks they doth protest too much.
Seems as though they are conceding that Anderson was less safe in Moultrie than in Sumter. And that is what really angered them - not that Anderson made a "hostile act" (which he didn't), but that when they were ready to make their own "hostile act" it would be much harder to root him out of Sumter.