You're mistaken. ISIS has employed chemical weapons against the Kurds, there is nothing incredible about al-Nusra having chemical weapons too. There have also been reported instances of the SAA suffering from chemical weapons attacks. Irregular forces in this conflict have demonstrated the ability to acquire and employ chemical weapons. Also, there's this: https://www.theguardian.com/world/2...s-found-in-mosul-in-isis-lab-say-iraqi-forcesOn the other hand the idea that the rebels have chemical weapon stock piles is just not credible.
Remember a lot of the rebel groups are just a smaller, weaker, version of ISIS.