Here's a hypothesis that seems to cover the facts we have so far: The report was a signal for a U.S.-based cell to do something with the nuclear material delivered on that Liberian-registered ship. When "the author" got word that the ship had been stopped, he issued the retraction to 1) tell the cell to forget it; 2) keep Osama's myth alive.
Of course, another fitting hypothesis is that the report was simply a glitch by an overzealous al-Qaeda content provider, who wanted to remind the world on this anniversary that they would continue their jihad, and in his excitement forgot the fact that we're not supposed to know Osama is dead.