It's amazing, isn't it, how you select a single sentence from an essay and use it to ignore the plain meaning of the entire essay.
I thought you might go for that sentence, and so I wrote the author of the essay himself, Juan Cole. I asked him if he would elaborate on that sentence and on the possibility that the document was forged. Here's what he said:
The document is an authentic al-Qaeda document. My speculation was whether
it was produced by Atta or by a multinational group in Kandahar and then
edited by Atta. It often quotes al-Banna almost verbatim. The FBI knows
bupkes about Islam and is completely unable to forge such a thing.So there is no doubt about the al-Qaeda origin of the Doomsday Document. Atta is the Arab without good style that may have edited the document, but without care, or he may have written it himself. There is no possibility in Cole's mind that the FBI, who released the Doomsday document, could have forged it.
You are stuck with this thoughtful analysis on how the 9/11 hijackers used their antinomian acts to convict their souls with guilt and thus steel themselves to the dreadful task ahead.
PS:
Who is al-Banna? Someone an Egyptian fanatic (Atta) would have deified. Since his works are quoted extensively, this could account for the "group" feel of the document, leaving Atta the sole author.