This is just the first publicly provable instance. I think the whole reason Dear Leader has been very careful not to name names with regards to the use of chemical weapons in Syria, is that he knew fairly early on that it was the Muslin Brotherhood "rebels"

that were using them, and the Muslim Brotherhood has been the horse he's been backing this whole time.
If he had had evidence that Bashir Assad was using nerve gas on his own people, he'd have been trumpeting it from the rooftop of the West wing before the ink had dried on the page.