Yeah, not to be all fucking dour about it but the witch would have grown up as a serf, meaning he would have limitations on what food he was allowed to take from the land. As well as being on a low income. Meaning that should influence his behaviour around food, which would have been scarce and a highly valued commodity. His fellow peasants could be sentenced to prison or even death taking any vegetables, fish etc if their landlord deemed it theft. So for the witch to not show any value in such a lavish meal seems out of character.
They live on their landlords estate so a bountiful crop is not something that could be hidden, especially when their fellow serfs are also restrained in what food they have access too meaning to see someone have such a big supply of food would cause resentment. The practical usefulness of the witches powers seems limited, they don't try to use their magic to prevent pain from torture or to try and escape when they are under captivity.
I agree, yes it's fantasy but it makes for a more credible plot line if the character behave as you would expect from their social/economic status and it has some historical basis. Earlier in the comments for example someone was saying all the characters must be unclean as they never appear to wash or change their clothes, despite historically the undershirts would be changed at least once a day and people of all classes would at least wash with water and a rag. It's those more mundane, every day details which make the plot more authentic, just as the reaction of the witch should be more fitting to someone who has the status of the serf, rather than behaving as though he has grown up being a wealthy royal who thinks nothing of skipping a meal because he knows he can eat later, or throwing food on the ground that a serf family would likely never have access too such as all the meat, sugar etc.
You're both going on like the witches are just "serfs", but it has literally shown in the manga that the witches' magic is capable of completely distorting the senses and even physically altering your sex... Logic doesn't apply to the witches specifically. If the same thing happens with a person that actually doesn't make sense, feel free and bring these thoughts up then, but this line of thought doesn't apply to the witches specifically. "witches who have access to magic don't value food?!" Besides, we don't know how they live and witchcraft and bountiful harvest are synonymous.
But... the food Σ(っ°Д °)っ