How does adding a current source and extra resistor to source follower improve its performance?
The current through the bottom resistor is set by the cut-off voltage of the bottom FET.
This current is basically the same as the current through the top resistor, so
the voltage drop on the top resistor matches the cut-off voltage of the bottom FET.
If the bottom FET is similar enough to the top FET, and the current through the  Vout terminal is small, then the voltage offset will be cancelled.