They probably didn't know it had Sarin. If they had, there's a good shot they would have known that it as basically harmless unless fired out of tube artillery.
"Isolated" doesn't seem relevant, IMHO. One big reason we believed Saddam had WMD's is that a lot of shells, etc., were unaccounted for. This validates that reasoning. Also, the fact that it took more than a year for this stuff to turn up suggests that we obviously have not found all their stockpiles, and that more of it is likely to turn up.
I'd like to see those numbskulls try to mix it manually, though. Lots of dead terrorists if they try that.
They do now.
Maybe a test to see if they work ?