Hummm, the wording does not specifically say that the "toxic gas" was contained IN the bombs, just that it was "unleashed".
Did the "leashed" "toxic gas" just happen to be sitting in the Rebel's buildings by coincidence before the bomb were dropped?
This sure beats the heck outa me.
Oh I think you are right on target. Makes more sense than Assad committing suicide when he’s almost won the battle. Twice now.