The account I read last week (which was admittedly short on details) was that ISIS was attempting to fill a shell with chlorine gas when it detonated. My interpretation was that the shell was an empty gas shell.
There were numerous reports immediately after the 2003 invasion of stockpiles of deteriorating chemical weapons being discovered throughout Iraq. A lot of effort was expended during 2004-2011 in locating and destroying these weapons. I suspect there might be stocks of empty gas shells that were either missed or exempted from destruction for reasons of practicality (too many of them).
Another reason to think this was a release of bottled gas is that there is no mention of other shelling. If, as you seem to suggest, ISIS can deliver gas by artillery (which requires specialized equipment, crews, and training), why wouldn’t they just pound the Iraqi Army positions with conventional artillery fire?