Misidentification
An IL76 would be a high value target, whether it were Russian government or a troop carrier, and they have quite a swept wing (albeit with 4 engines on).
I'm guessing the guys who fired didn't understand the capabilities of the system and were 'funnelled' into a firing process without taking the time to even consider what the target might be.
Plenty of people in plenty of anonymous government buildings knew the 'separatists' had claimed to have acquired a GRIZZLY. Apparently nobody considered the implications for civilian traffic, or didn't make their point clearly enough to those in charge of NOTAMs?