That's a bad example because France isn't really an ethnicity. It's a group of regions that used to speak different languages even after World War II.

You have Bretons that are ethnically close to Irish/Scotish, Alsaciens who are ethnically Germans, Basque who are similar to the Spanish Basque, Corse who want to be independent...

A better example of a country that more or less encompass an ethnicity would be Germany, or Japan. But even there you'll find exceptions (Ainu in Hokkaido, Okinawa being also ethnically different as Ryukyu vs Yamato...)

French is indeed an ethnicity in that there's a shared set of values, cultural traits and genes that you can distinguish from, say, the Greeks.

The fact that it has substructure and blurred lines doesn't mean it isn't a valid abstraction.

Some Basques may want a word, and some Corsicans, Bretons, Alsatians, Catalans and probably others as well.