This falls into a common trap. Because we cannot succinctly define a salad in one sentence we decide that it cannot be defined at all. This argument effectively reducto ad absurdums itself by coming to the conclusion that all foods are salad.
If we start from a position where we discount nothing from being a salad, and we have only salads (and soup, seemingly) to base our analysis on, how can we ever identify the boundaries of salad? The whole argument is based on the flawed premise that anything could be a salad.