They provide a link to the section where they elaborate on "commit first vs test first", here is the relevant text
Instead of jumping straight to the commit step, Fossil applies the proposed merge to the local working directory only, requiring a separate check-in step before the change is committed to the repository. This gives you a chance to test the change first, either manually or by running your software's automatic tests. (Ideally, both!) Thus, Fossil doesn't need rebase, squashing, reset --hard, or other Git commit mutating mechanisms
Even if you don't have a special setup, having a section telling you that is still a helpful thing to quickly assess a new project.
I appreciate knowing that a project should Just Work with minimal setup so I don't have to guess or make assumptions