The Grbl feature is unrelated to the X-Controller feature.
Grbl Step Idle Delay turns off all the stepper drivers a set number of milliseconds after Grbl enters idle mode. We do not recommend using that on X-Carves. It is best to leave the motors on or position may be lost. X-Controller does support it though.
The X-Controller idle reduction feature is different. It reduces current rather than stops current. It is also independent for each axis. The X and Y can still be moving and the Z could enter current reduction. Stepper motors lose torque with speed, so even if they have reduced torque at idle, they may actually have more torque than the running, full current, motors. This feature results in lower average motor temperature, so higher full current settings can be used to improve performance.