I don't know for previous versions but in Blender 5.1, I've had an extra step to do to get the ui script to be executed automatically when opening my file:
in the text editor on luna_rig_ui.py, enter the 'Text' menu and enable 'Register'.
It was not e feature targeted in the video but it matches what one would get using Rigify (and avoid having to access the script to run it manually every time the file is loaded)
