Since the rebels have no aerial means to deliver chemical weapons, your theory supposes they were producing and/or storing chemical weapons procured elsewhere with the express intention of giving the storage location to Assad's forces, then hoping Assad would order a strike on that location and kill innocent civilians, including children, in order to capitalize by turning world opinion against Assad. Any other use of chemical weapons by the rebels would define them as terrorists and turn world opinion against them. The first of these explanations stretches the imagination, while the second would just be poor strategy.

Assad had plenty to gain from a strike like this by making it clear to the rebels that he once again can and will use chemical weapons to kill them and their families in a horrible way, thereby dissuading the rebels from continuing to oppose him. We know Assad has had and used chemical weapons against his people in the past, supporting this theory.

Assad did it.