The English is only in snippet view, but I found it in the French - yes,Thiebault is vague about when it happened, but he would seem to be ascribing it to the summer of 1806, and definitely to Berthier. He doesn't actually write as if he was an eye-witness, I suspect the story was revived about that time.

I still think some other writer mentions it, the duchesse d'Abrantes perhaps?

regards,

Susan