Assad had nothing to gain from mounting a chemical weapons attack on civilians. Quite the opposite. Just a few days ago the White House announced that the US was no longer trying to get rid of Assad. That's a highly positive development for the Syrian government, coming on the heels of other positive developments such as the retaking of Aleppo and the losses that Islamic State is experiencing. Hence, attacking civilians with chemical weapons would be a truly stupid thing to do at this point. (Much like the suspicious 2013 attack that happened when UN chemical weapons inspectors were located just a few kilometers away.)

On the other hand, hearing that the White House had stopped opposing Assad would be very bad news for the rebels in Idlib. Wouldn't it be useful to them if there were suddenly a big propaganda story they could use against the Syrian government?