It is circumstantial evidence.
The difference between the US having them and Iraq having them ready to use is that the US had them ready to use because we were at war with Iraq, who was known to have chem/bio weapons.
The same conclusion cannot be drawn by US having them because the idea that Iraq had them ready to use because they believed that we would use chem/bio against them is not believable because it was clear that the US had no intention of using chem/bio weapons.
So for Iraq to have them at the ready was because they were prepared to use chem/bio weapons, not because anyone seriously thought that they may have to protect themselves against us using them.