In fact, my favorite author Dostoevsky (mentioned in the article) goes out of his way in his book Demons (click on my name) to point out that many atheists believe in God more than priests--they're just quite mad at him about something.
Anyway, the point I am making is that the claim that Objectivists make (and other libertarians) pursuant to the idea that they have "logically derived" their ethical code. Nothing could be further from the truth. They're down there with the relativists.
But, as the author of this article so well notes--everyone is an absolutist about his own version of ethics...
Actually, most of the atheists I know are not Objectivists epistemologically, though they may agree with some of the basic principles. Most atheists I know have their epistemological foundations in "pan-critical rationalism", which is actually credited to a William Bartley, a respected protestant theologian who did a lot of much-referenced work on rationality in Christianity. While Bartley wrote from a Christian perspective, his reduction of rational Christian epistemology is equally applicable to the atheist as well.
It is a rational moral absolutist framework, but it also eliminates the necessity of a fixed reference for morality. In doing so, many of the lingering irrationalities of the Christian epistemological premise could be resolved without eliminating God as a valid reference for morality. But as a consequence of this, he acknowledged that it was clear that God was not necessarily the only valid reference for a rational absolutist framework nor was any "God-like" figure required. I'd suggest reading the book ("Retreat To Commitment" IIRC), as it is hard to do justice to it here and a significant fundamental advance in philosophy when it was written. It paints a more powerful picture than the caricatures that most people argue over.
Epistemologically speaking, pan-critical rationalism is frequently considered the "state-of-the-art", whatever that means in the field of philosophy. :-) It is curious that both Christians and atheists have been able to adopt its epistemology without contradiction.