I would imagine that the likes of the SA22 are there for close defence against incoming rocket and artillery attacks, although something like the Chinese LD2000 would probably be far better suited to that role.
If the Russians were on the ball, they should have also deployed tracking radar with their artillery that should give them near instance origination co-ordinates for any incoming ordinance for them to deploy near immediate counter-battery fire.
My guess would be that ISIS fired some rockets at the general vicinity of the airbase, but the rockets didn't really come close to hitting the base itself, and may well have taken some Russian counter-battery artillery fire for their troubles.
Insurgents and terrorists could launch mortar attacks on western bases because mortars are fairly light weapons easy to smuggle close to their intended target. Mortars are also fairly short ranged, so against a large airbase, its hard to miss the base entirely. But Insurgents generally lack the training, discipline and co-ordination to do any damage as they are just effectively taking blind pop shots hoping to get lucky rather than aiming for specific targets.
I seriously doubt ISIS could get grads to within visual range of the Russian base without being spotted and engaged first, so they would almost certainly been firing from long range.
The grads were hardly precise weapons to start with, so the further back they are firing from, the less likely they would even hit the airport.