I'm not so sure. Even a relatively small amount of Sarin can be VERY lethal to a wide area, so the Daesh wouldn't have needed a lot of it for it to have that kind of effect. On the other hand, NOT having a lot of it would adequately explain why they never used it before: if they only have enough of it to use it once or twice, they would have been saving it for a "last stand" if it looked like Assad/Russia was really about to deliver a killing blow. They might have enough of the stuff for one or two attacks that could, in a pinch, turn the tide of a losing battle, but not enough to start using it at random to win meaningless skirmishes in cities they don't really even control.
They certainly have less of it NOW though.