Usually when people say "gender is a social construct" they mean that when your 7th grade biology teacher told you there are males and females, there was more to the story.

If someone tried to tell you that you can fit everything into either the "food" bucket or the "not food" bucket, you would definitely have questions. How do we define food? Food for whom?

If you define food as something edible, with nutritional value... How much nutritional value? A stone covered in lichen probably has nutritional value and I might try to eat it if I were starving to death. Raw grains of wheat might have nutritional value but also might make me sick of I ate them whole, but we all recognize they are generally edible and are usually processed into bread.

"Food" is a useful concept in that it can usually help guide humans toward sustenance. It was never meant as some airtight philosophical concept.