The input mesh is internally treated as a Catmull-Clark subdivision surface so it is possible that parts or everything of the original shape is modified, in particular when triangle primitives or features like creases are present in the input mesh.
You could preserve the cube by "tagging" the edges of the cube as "hard crease edges", however, it seems that you only what to modify the input mesh itself by applying displacements, right? Or you might want to tessellate the input geometry without actually applying any subdivision rules at all?
In that case it might be "easier" to do all tessellation and displacement of the input geometry in your application and just transfer all generated geometry data per frame to Embree (as a standard triangle object).
I guess the DX11 example does not use any subdivision surfaces at all but basically tessellates each input primitive (triangle,quad) based on the edge levels and then applies the displacement function. Our "subdivision object" in Embree implies Catmull-Clark subdivision so you will need hard crease edges to preserve the original shape and therefore emulate your use case. A future version of Embree could support this "planar tessellation" case more efficiently (needs discussion within the Embree team). Another option would be to do the tessellation + displacement entirely in the application and just transfer the geometry to Embree but that's probably too inconvenient.
It's an interesting use case for sure. Could you give us a bit more details (here or offline) for what you need the displaced surfaces exactly?