Here is the only relevant bit in the T&Cs: "Complete your eligible booking at
aircanada.com with one of the promotion codes and you will receive complimentary Air Canada Cafe items during your journey. This travel option is applicable only for new reservations completed at
aircanada.com and when travel begins at least 72 hours after time of booking."
One could argue that the FP booking is indeed made at aircanada.com, but one could also counter-argue that "with one of the promotion codes" means that the booking must be made together with the code.
I agree with the point that this is poorly implemented and I doubt that this was intentional to reduce the functionality of FPs. But then, if it is an IT limitation, perhaps this could have been addressed by offering the phone-in option to add the code (since it simply involves adding LT07 to the PNR and cancelling out the code used). And considering that there is an ability to add travel options after the booking is made, enabling a promo code add-on seems like a no-brainer to me. Perhaps I am expecting too much though.