This is not correct. If you are a FB platinum member and you are on a national flight in France, you only have lounge access when departing from CDG. You will have lounge access to all lounges on your itinerary, whenever you are on a same-day international ticket.

This seems to be true throughout ST. For example, if you're a DL gold or platinum, you will have lounge access on those days that you also have an international connection, but not on other days (even if you're on an international itinerary).