So Lincoln's pledges in a speech to hold onto a fort are official policy written in stone? Nice try but it simply doesn't work that way. Lincoln knew that the south, with good reason, would resist his efforts to resupply by blatant military means. He knew that the south, with good reason, would doubt the sincerity of the northern government especially if it acted unilaterally. It was Lincoln's adamant, uncompromising, and outright hostile inflexibility that started a fight at Fort Sumter. He essentially said to the south "No negotiations. No diplomacy, formal or informal. No way of doing any of this on any terms other than mine. Heck, I won't even meet with you to discuss any of our differences. Either do it my way or the highway."
Lincoln went in there expecting that everybody who differed with even so much as a minute detail of his approach would bow down and abide by his so-called authority over states that were not under his control to begin with. It didn't matter how much of a threat his actions presented to the south or how many people would potentially die as a result of his reckless inflexibility. Lincoln steered head on into a war, the consequences be damned. And with that kind of Saddam Hussein-esque attitude it is little mystery why the south reacted to him as they did!
In retrospect perhaps he should have known. The firing on the Star of the West in January should have been a good indication of the southern lack of interest in a peaceful solution. Southern hostile intentions were later made evident when they fired on the Rhoda Shannon merely because she flew the Stars and Stripes, but it's doubtful that Lincoln would have been aware of that incident coming, as it did, about a week before the attack. Still Lincoln held out hope that if he made his it clear that he wanted to land supplies only then Davis wouldn't attack. As it turned out Lincoln was mistaken.