No way to "know" for sure, but the circumstances argue against that. Based on the way it was used, the guys who set it probably did not know it was Sarin. Absent the rotation imparted from firing it from artillery, you don't get the mixing necessary to activate the stuff.
If they believed it to be just an ordinary artillery round, there's no need to go to Syria to get one of those. They're stockpiled in lots of places in Iraq. The facts suggest these clowns have some old Iraqi bunker with some of these things, and grabbed this one, and set it up without knowing what it was.
If you assume that it was a known WMD imported from Syria, you'd have to assume that the people who provided it know something about it, and would instruct the recipients as to its employment. But that doesn't square with the manner in which it was employed.
The fact that this apparently wasn't intended to be used for the sarin gas, it suggests to me that these have been hidden in plain sight. Unmarked shells containing WMD may have been randomly stored amongst the non-WMD shells. I pray for the safety of our troops.