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