The issue

After noticing that my action run looked successful despite my pathspec not matching any files and this action even logging an error, I read the "Adding files" section which at the time of writing reads

The action adds files using a regular git add command, so you can put every kind of argument in the add option. For example, if you want to force-add a file: ./path/to/file.txt --force.
The script will not stop if one of the git commands doesn't match any file. E.g.: if your command shows a "fatal: pathspec 'yourFile' did not match any files" error the action will go on.
You can also use JSON or YAML arrays (e.g. '["first", "second"]', "['first', 'second']") to make the action run multiple git add commands: the action will log how your input has been parsed. Please mind that your input still needs to be a string because of how GitHub Actions works with inputs: just write your array inside the string, the action will parse it later.

Especially the highlighted sentence made me believe that's just the intended behaviour and I have to deal with it. And I stopped looking further. Someone else wrote the workflow, so I never studies this action and only later noticed the pathspec_error_handling input.

Suggestion

I think if the documentation mentions the option to override the pathspec error behaviour, this would make it clearer prevent similar cases of confusion. I'll submit a PR for this.