According to the New England Journal of Medicine, the most likely source of Stevens' exposure was the so-called "J-Lo" letter. Published reports are sketchy and somewhat contradictory, but suggest this package was received about a week before 9/11, set aside as "kook mail" until later, and Stevens was exposed to the contents on 9/19. Even if the reports of a delay between the package's arrival and Steven's exposure to the contents are accurate, it is still not clear whether the package was opened on arrival, or whether it was simply set aside in the mail room unopened, until it piqued somebody's interest.
Seems kind of iffy to me. The reams of paper are sealed, by more paper and some glue. I could see the spores settling onto the outside of the reams, but not much getting onto the sheets on the inside, there to be "aerolized" by the fans and other moving parts. The wrappers end up in the trash or recycle bin, not in the copier.