I'm skeptical for a very simple reason. There would have to be pretty widespread knowledge among the Iraqi military that WMDs were being used in some areas.
If that were the case, we would have several sources comming forward with that information now, and their accounts could be compared to give some level of confirmation as to their accuracy.
To the best of my knowledge, that hasn't happened.
I can believe that the weapons were smuggled out of Syria.
I have a harder time believing that they were used in the conflict like this article suggests. That simply requires too many people knowing too much for it to remain secret this long.
Skepticism is a healthy thing;however, if you were part of any army which had used WMD-and which had been warned of very consequences-would YOU be the first to volunteer such information?