How does this heavily oxidized copper-to-steel joint work in earthing?
The point of the clamp and crimp connections is to make gas-free joints where the two parts are forced against each other.  People have been making grounding rods for a long time now, and I expect they know what works and what doesn't.  Very likely the electric code also requires certain things be done, again based on history and experience.

Are you sure the rod isn't copper?  The clamp looks like it might be brass, with the wire and rod copper.