Well for personal reasons I'm quite knowledgeable about French aristocracy history & whereabouts, especially prior to this sad event (the Revolution), I have never heard of that story and I very much doubt there's any truth in it, mostly because of the way aristocratic families recorded the births and genealogy of their offspring.
But then again, Brits are used to saying a lot of stupidities about us "frogs", out of jealousy most of the time: they dream about spending their holidays or buying a house in France, eating French cheese, drinking French wine, flying French planes (Concorde) & kissing French girls!
MF