Yeah no, not buying it. The people reporting it, mainly Hadi Abdallah and other local sources have strong ties to HTS, previously Nusra Front (Al Qaeda in Syria) and not to mention Al Jazeera who is vehemently pro rebel, a Qatari and Nusra mouthpiece and lacks any sort of professional neutrality in reporting. This is deja vu, every time they suffer a defeat they orchestrate a propaganda piece to justify their defeat.
HTS launched an offensive last week on northern Hama violating the Asetana COH, while HTS was not a signatory all the FSA groups fighting with it signed the deal. They thought they would surprise the garrison at Hama and take the prized city. Now they're suffering an embarrassing defeat and are looking at saving face.
All the chemical stockpiles the SAA had were destroyed by the UN and the UN is monitoring closely. Why would the SAA drop chemical weapons on an isolated civilian target after they routed HTS when they could've used the chemical weapons last week on massed HTS infantry while the SAA was on the defensive and loosing ground?
Khan Sheykhoun is several kilometeres away from the nearest SAA position, if the SAA wants to enter it, it would take months and hundreds of soldiers dead. If they wanted to wipe the city off the map with gas, they're going to need a lot more than a single payload.