Iraq did apparently work on weaponozing anthrax.
However, anthrax doesn’t have to be weaponized to kill. All that is required at the most basic level is for someone to get their hands on a virulent strain. The so-called Ames strain that was used in the 2001 attacks is only one of several possibilities.
It is the nature of the bug to spread through aerosolization - that is it’s normal life cycle. Sure, it could be a lot more effective as a weapon with special processing techniques and additives. However, I haven’t seen any definitive evidence that this was the case here.
There is plently of open-source material in the scientific literature to culture the bug. This is why bioterrorism ought to be of acute concern, because at least at the small-scale level, it is so easy to do. In fact, I think that was the the motivation of who ever did it - to point that out. Unfortunately for all concerned, it got away from the perp and did a lot more damage than he/she/they probably intended.
That’s not to excuse the perp - but understanding the motive is, I believe, a key element in identifying the actor.
Key to that is why Daschle and Leahy, out of everyone in the US Government, were sent letters, IMHO.