I would like to expand on @Crantisz good answer, if I may.
The issue you are facing is that the tutorial is generating its textures using the Cycles material nodes, but putting the output into the "displacement" slot of the Material Output node using the Microdisplacements feature. As I understand it, this means that the sphere's geometry is actually getting modified at render time--it's not just lighting effects as you would get with a normal map--but there's no obvious way to "capture" what is happening to the geometry.
It seems to me that what the "RockGenerator" node group is outputting from its "Displacment" output is essentially just a bump map. So I suggest baking that bump map to a texture, which you can then use to displace the geometry of a sphere with a subsurf or (as @Crantisz suggests) multires modifier. You can then use a "Displace" modifier on the sphere, using your bump map as the input. Once displaced, you can use a combination of baking a normal map (at high geometry levels) and then bumping the "multires" to a low level and applying the modifier for your base geometry.
In doing the texture baking, I suggest putting the output of the "RockGenerator" group into a "Multiply" node and then into an "Emission" texture. Without the "Multiply," areas of texture get clipped (I multiplied by .2 just as an approximation, maybe a more scientific approach could be used). We need to essentially "normalize" the bump map, but I do not think Cycles material nodes have a normalize function, hence the "Multiply." You could use a color ramp to try to get the closest approximation.
This screenshot shows what the geometry looks like with the baked bump map displacing the geometry of a sphere with a multires modifier:
From this point, you would still need to bake a normal map, and also apply the multires modifier at the desired level of subdivisions.