Incorrect. If F is full, the service you will receive is less and slower as if F is not full.
Even if you pay fullfare F, a platinum member upgraded from C will receive better service as you as that guy will be asked first about menu choices/shower times, etc.

Besides, if F is full, and you tell them that you want your lunch/dinner later in the flight, you will open a can of worms. In this case the meals that cockpit and crew can eat becomes quite limited and they will push you to make your choice NOW, even if you want to eat later. If you want to win the "worst passenger of the year award by EK cabin crew", then just say "I will eat one hour before landing but I really cannot tell you now what I will be hungry for then. I expect you have all the choices available still" -- )

Of course, this is not EK specific.