That video is 2 years old when KiCad 5.1.2 was current, so the references are going to be out of date. The vague reference you are referring to was supposed to be this thread “on the forums”:
Which links to the same spot as the link in your EDIT above. So you are at least on the right track now. The
spice.rc file needs to be created as a new blank text file.
As for updating the ngspice that ships with KiCad 5.1.9 (ngspice-32) and if you really need to…I would say it’s worth it but it would be better to uninstall KiCad 5.1.9 and simply install KiCad 5.1.10 which already ships with ngspice-34.
The reason why Holger’s #kicad download link is not currently working is because it was temporary. It was for giving us options to test pre-release builds of ngspice, which fixed critical bugs, along with KiCad. Now that the latest ngspice-34 is available through the standard update process (found in Step 1 of the link you found), the #kicad specific download link is not needed until there are new pre-release builds he thinks will be useful for end-users to test or which can’t wait until the next official release cycle.