The accuracy may not be that poor.
I can't speak specifically for the S300, since I do not know how it works in the SS mode. However, I can speak to how a generic command guided (CG) SAM might be used in an SS mode.
From open sources, including the Australian Airpower article I listed before, the original S300 missiles, the 5V55K and KD models, are described as CG. Later versions apparently added additional capability.
For those not familiar (
buffnut453, I assume you know this, I am describing it for others that might not), CG missiles are basically radio controlled missiles. The radar tracks the target of interest, and then it uplinks (sends) commands to the missile to guide the missile to the position in space that the radar believes the target to be at. The missile does not guide itself, the radar on the ground tells the missile how to correct its path, and the ground radar guides the missile all the way to the target. It is, by design, accurate enough against a flying jet to get the missile within the kill radius of the warhead of the missile. For some CG missiles this might require putting the missile within ~25 feet of the moving target in order to be lethal.
Because the ground radar guides the missile, and radar track error naturally get larger with range, the CG system has practical accuracy limitations based on range. You don't use this technique for very long range shots, because at very long range the missile miss distance (based on track errors), on a moving target, might exceed the kill radius of the warhead.
Now lets apply this technique to a non-moving ground target. The radar does not even have to see, or be able to track, the target, all you need is surveyed numbers from the radar to the target, i.e., the target is 100.2 degrees azimuth true, 0.4 degrees elevation, and 50.4 km away. If the missile is launched in an up and over trajectory, the radar will provide guidance as if it was tracking that unmoving target at those coordinates. It will continue to guide the missile as long as it can see the missile. In an up and over it (the radar) might be able to see the missile until the missile is within a few thousand feet of the target, and the last command would arrive with the missile pointed right at the unmoving ground target.
For an unmoving ground target the accuracy might be better than for a moving flying target, particularly at longer ranges.
T!