Ah right, that one. I had seen the video already before.
So if one makes use of custom packages like this, it can mess up your projects for other machines... which is good to know and something I hadn't realized until now (not that I had really made use of this way of doing things so far...).
It might be worthwhile to check out if Unity can add it with a relative path if the custom package resides in the project's root (so one abofe `Assets` or even one further up if one puts one more folder level from the git repo's root).