I’m fine with their first consideration:

> A salad must be a mixture of greens.

The only additional realization you need is that things like “fruit salad” and “chicken salad” are using the word “salad” figuratively. There’s no reason to expect our definition of “salad” to cover figurative usage. After all, do you really expect to cover, for instance, “word salad”?

> A salad must be a mixture of greens.

My mums German potato salad disagrees

Reminds me of when I ordered Salade Marché in Paris and it was a large slice of paté with a frisson of endive on the side :-)

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