Don't know whether you can help but here goes. I have just taken on a rockery, which hasn’t been forked for at least 18 months and is like turning over concrete as one would expect would you recommend mixing in some topsoil or compost?
Rockeries are usually a mix of soil/compost and lots of grit making it very free draining. Are you sure the previous owner hasn't cemented the rocks in? You never know!
lots of small stones in there but need to clear all the plants out of it as well