The author runs his tests after his partial commits. He does this by stashing the remaining changes, testing the newly-committed code, and then continuing to make partial commits. Here's the example in the article:

        % git stash
        % make test
        ...
        OK
        % git stash pop

Also keep in mind that the initial tree contains WIP commits that are unlikely to work or pass tests, so reordering commits can hardly make things worse.