My problem is that for unrelated scripts, the git model suggests to me that each script would have its own git repository. Although this is obviously possible, I wonder how managable this would be in practice.

(emphasis by me). Suggestions are just generally good ideas; it doesn't mean that you have to follow them, if they don't apply in your case.

I often had the situation of having a repository with various stuff. Over time some of the scripts grew into larger projects, so I extracted them (and their history) into a separate repository, and deleted them in the original repo, leaving a note on where to find the files now.

More repositories imply more administration overhead, so I only go the route of more repositories when I see a real benefit from it.