Ummm... Most atheists I know are moral absolutists in the strongest sense. Somewhere along the way someone in the non-atheist camp concocted the idea that atheists were moral relativists (perhaps from an outlier example), but any real interaction with them makes it very hard to construe them as this. In my experience, they are statistically more prone to being moral absolutists than self-labeled Christians, for whatever that's worth. You got your idiots in both camps, but most of the atheist position is premised on moral absolutism, so you would pretty much expect to find it there whether you agree with them or not.
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...