Our did learn that it was not acceptable for her to eat of the others plate. It did take a couple of months of being alert to what she was doing and correcting her. She did learn though. Funny thing was - after she learned, they both ate their food - then switched and licked up any small little scraps from each others plates. We let them continue to do that. They also by that time had decided they were best friends and were bonding and starting to watch out for each other.
I agree that it isn't food aggression. You would get a violent reaction, growling, snarling etc if anyone comes near the food, or fighting with the other dog to take the food away.