I figured this was him just appreciating the food here in mariejoa not necessarily Ryokugyu cooked it
I had this doubt too, but makes no sense for Fujitora to thank him for the food
But it could be either:
-Fujitora thanked GB and praised the food cuz he was the one who cooked it
or
-Fujitora already knew GB was fasting to begin with, so he told him to try eating the food at Mariejoa because it was decent food, GB didn't cooked anything in this case.
My theory is that Greenbull's powers have something to do with him turning into a cook (maybe he perfoms photosynthesis?), thats why he fasts for so long, consequently these powers made him look for good food throughout his life, so that would light up his interest at eating again, forcing him to be an awesome cook looking for food good enough for him to eat.
It's like the opposite of what Sanji stands for, Sanji feeds the hungry, Sanji himself fasted for several days alongside Zeff, Sanji even missed eating rotten food after that event, he ate Cosette's food from the floor and praised it, on the other hand Greenbull chooses to fast by himself, even if there is pretty good right in front of him.
Greenbull is someone that would just be pleased by the cake Sanji baked to Big Mom, know what im saying?
Their fight could even end with Sanji making a dish for GB while he is on the ground basically defeated, Sanji feeds him and GB looks amazed by how good it is.