" if the nonce range is small enough (I think less than 2^248?), it is trivial to derive a solution for the private key"
how we can derive a solution for the private key in this example? I have no idea
I wrote some equations for the private key which you can view
here. In particular, the first one which calculates the private key from r, s, and z (in the post it's called h) will be useful.