As I understand it (also neither a mathematician nor French), it's a parody on a certain type of math paper, I'm guessing in abstract algebra, where the author

As I understand it (also neither a mathematician nor French), it's a parody on a certain type of math paper, I'm guessing in abstract algebra, where the author does nothing but present a theory that is a "generalization" of some other theories, with no concern whether this generalization is helpful in any context. So you get a "mustard watch", a thing which behaves simultaneously as a watch and as a mustard container, and finding a situation where that would be useful is an exercise for the reader.