More information coming from our French accountant:

I asked to one of my colleagues, he said it's normal that the letter was not
sent with "recommande". So, there's nothing to do. Mr Thiery's lawyer has now 3
month maybe 5 to make the complete file, then you'll have 3 month to
answer. Then the court will take the decision (maybe in 1, 2 or 3 years...). If
they decide the Cour d'Appel was wrong, there will be a new court who will juge
the case. Good luckSorry, I would have prefered to tell you some good news.