... though now that I see it was artillery shell (first time I read the story it was saying a roadside bomb) I'd have to agree it almost certainly IS from inside, or at least a syrian import. Al-qaeda (etc.) has been building a good deal of capability in chemical weapons as bombs but not as far as I know as military equipment, so certainly looks more like ex-saddam material.
Please forgive me. I did not know what your stance was on the issues. Now that I know where you are coming from, I can and do respect you for asking the question.
We don't know where they came from, but if neighboring countries have these weapons, then it is very likely that Iraq had them, maybe not for terrorism, but self-defense. So I believe that it is very likely that they are from Iraqi stockpiles.