I think the implication is that the 3 who are eating didn’t know what they were eating, only that the food was prepared. The guy finds a creature (a frog, i guess?), thinking a frog got into his dish, but the cook reveals that the creature IS the dish, which the others find immediately concerning and disgusting.

I always interpreted the creature to be some unknown, unrecognizable thing from the swamp, which makes the idea of eating it even more repulsive.