Excellent observation. I'd missed it, but it's very crucial...
"Usama bin Laden indeed believes that he is the Mahadi, but, he cannot proclaim that himself.
The Quran states that anyone who claims to be the Mahdi, is nothing but an imposter.
bin Laden, in carefully orchestrating the psychology of his appearances and statemetns is fostering the public perception that he is indeed the Mahadi and that the forces of the black flag should rally to his cause. But, again, it has to be popular exultation and not his own proclaimations."
The Koran quotes the baby Isa ('Jesus', but it looks more like Esau):
[19.29] But she pointed to him. They said: How should we speak to one who was a child in the cradle?
[19.30] He said: Surely I am a servant of Allah; He has given me the Book and made me a prophet;