Seat assignment is not guaranteed. AF changed their seat assignment policy and you have no recourse if they decide to move you to another seat if you do not wish to pay. The tweeter guy gave you some assurance and that is nice. Whether you will automatically be reallocated seats by the system shortly before the flight is an open question.
Whether those duo seats are truly attractive depend on pax. They are very noisy from the reactors noise and proximity to the lav and area where pax congregate to chat.