There's nothing wrong with that sentence. It's totally true. Silica DOES help prevent anthrax from absorbing moisture and aggregating, which DOES help make anthrax easier to aerosolize.
Mullick is stating a fact about anthrax in general. She didn't say, "The anthrax was put into the powder to keep the anthrax from aggregating." She's confirming that she was MAKING AN ASSUMPTION by making a general statment and implying it was true for the attack anthrax.
Ed
I suppose the drugs you are taking will also help reconcile your statement "She's confirming that she was MAKING AN ASSUMPTION by making a general statment and implying it was true for the attack anthrax." with the first two sentences of the AFIP report? :
When US Army investigators at Ft Detrick, Md, examined anthrax found in a letter sent to Sen. Thomas Daschle last fall, they discovered that the highly refined spores floated in the air, making them much easier for potential victims to inhale. What made this anthrax so easily aerosolized?