They are assuming that pain is a reaction to some sort of stimulus. In this case, of course living things with some sort of nervous system will feel pain, as they react when they are hurt, but one could argue that hitting a stone very hard could deform it, cause it to wobble, create cracks, etc. and that could constitute as pain.

Of course, this really stretching the definition of pain by a lot. Personally, I don’t think stones feel pain.