His comme this is not wildly used in cases where it gives you an advantage? (such as positive discrimination when entering a university, or for grants,...)
That's not really true. You wouldn't be actually able to define yourself as Afro-American in the US, beyond maybe lying on a form (and even that designation would probably be overridden if it mattered). Pretty-much everyone would consider your self-designation to be annoying BS.
A few full-blooded white people have tried passing as Afro-American (only possible because of the lingering effects of the one-drop rule), but they received a lot of press (and rejection) when the real story came out.
Please see https://www.census.gov/topics/population/race/about.html, and especially "An individual’s response to the race question is based upon self-identification. The Census Bureau does not tell individuals which boxes to mark or what heritage to write in."
The "annoying BS" you mention is people's response to that self-identification, not a legal one.
We do not have in France any such legal construct (this is even forbidden by law).