Such a check would make the function slower, which is a very bad idea regarding code efficiency. The function has been made private so that the user should not call it. Thus, it is the developer that needs to make sure that the `normalized_design` argument is correct.

As already mentioned I do not like the fact that this function computes a F-score for every use-case: It was originally designed for speed and was actually not a function (it became one for code readability). One comment of the PR was that for the sake of completeness, the function should also be callable with non-normalized designs. The latter change did not change the speed of the function but added a lot of code (that is never used in nilearn currently).