With your example only 20 faces are set, those facets become like the polygons or divisions of the rock model. 20 faces cannot show detailed displacements.
Give the rock a lot more faces if you want to see fine displacement. Maybe 256 or 512 faces gives you something like a sphere to start with. It will take longer to generate however if you use a population. The rock object is kind of an early Terragen left over, as I understand it, not highly developed, at one time it was considered to be dropped from Terragen, I believe.
Probably it is much better to import your own high polygon rock model and set it to use forced displacement.
Forgot to add there are subdivide mesh and subdivision level parameters in the rock. You probably will want to use those as well for more detail.
Made a test file and a couple renders of it.
1. Has 256 faces and subdivided 3 times
2. Has 256 faces and subdivided 4 times
Not much difference unless you look close or get the camera closer.
This uses one Pf internally in the rock node with color and displacement to the surface. (Tried using a Pf as an external Mesh displacer but it just didn't work very well . I think 256 is still not enough faces for that)