What convinces me is that it was a rebel stronghold. There was no positive for the rebels to unleash chemical weapons, and a lot of positives for Assad.
Are we talking Rebels, or are we talking ISIS? Also, if we're talking Rebels, can we rule out ISIS having picked some WMD's up and pulling a false-flag on Assad?
With Russian, Iranian, and I think some Chinese backing, Assad doesn't need WMD's to subdue an area. He can just bomb
or otherwise bombard it into oblivion using conventional arms. All using WMD's would do is bring down international condemnation upon himself.
The joys of fog of war.
The other thing is, if ISIS is involved, as victims or attackers, if not both, nothing can meaningfully be ruled out. This a culture that is known for suicide bombers and suicide attackers. So their view of the "righteous muslims" who were slain by that attack/event is likely to be more than a bit skewed. They died for a glorious cause, and will be richly rewarded in the afterlife.
Compounding things further for that matter, there is no guarantee that kind of screwed up thinking isn't also pervasive within the Rebel camps as well.
So it goes back to: It doesn't make sense for Assad to use the stuff, while the other groups involved in the fighting could very well have rationalized their way into using it, even on their own people.