Wesley Wark propagates a dangerous assumption based on zero evidence  The assumption is that "Assad did it." One might forgive that kind of carelessness if there weren't so many compelling reasons for doubt. But the list of counter-arguments is so long that there isn't space here to include them all, but here are some in no particular order.

1. A designated terrorist organization, al-Nusra, controls the area of the alleged attack.

2. Al-Nusra has already been accused by the UN of mounting chemical weapon attacks.

3. Al-Nusra is losing the civil war that they once were driving. Short of outside intervention, there is no hope that they will take over Syria now. 

4. The claim of an attack comes from social media, not from an independent investigation.

5. This would be the first time in the history of chemical weapons that they were delivered by jet aircraft. Helicopters are the typical vehicle used in these attacks. They carry more and the bulk isn't a problem.