You are quite precisely correct.  The Syrian Air Force hit the target with conventional weapons. All involved expected to see a massive secondary explosion. That did not happen. Instead, smoke, chemical smoke, began billowing from the site. It turns out that the Islamic rebels used that site to store chemicals, not sarin, that were deadly. The chemicals included organic phosphates and chlorine and they followed the wind and killed civilians.  There was a strong wind blowing that day and the cloud was driven to a nearby village and caused casualties.
https://gosint.wordpress.com/2017/04/07/former-dia-colonel-us-strikes-on-a-syria-based-on-a-lie/