The truth is that all the evidence is circumstantial, and no individual piece of evidence is definitive. But the totality and consistency of all this evidence points to a connection between the 9/11 hijackers and the anthrax mailings. (Plus, there is the total lack of evidence for any other theory regarding the anthrax mailings. None of the other theories that have been proposed are backed even by circumstantial evidence.)
So perhaps he chose the middle route, the cryptic signature, to be "read" only in the event of his demise after his name became infamous.