Sorry I didn’t find the key difference between retargeting in the video you have shown and the usual IK retargeting. In the solver? In the IK Goal? I have done the IK Rig, added chains with proper names and created a retargeter from the Mannequin IK Rig even before your video, and everything works except scale. Here is my IK window:
I also tried to set the “Retarget Root” into the “root” or “pelvis” bones, with the same effect. Can you explain what am I doing wrong?