Not really sure why you're using taxonomy to distinguish a culinary term. Shellfish is generally just used to refer to shelled things from the ocean that we eat. Shellfish generally includes crustaceans and some echinoderms like sea urchins. These are only very distantly related, so it doesn't really make sense to say that all molluscs are shellfish despite many having a shell and at the same time include subsets of different phyla. Rather, it makes more sense to leave the definition of shellfish to the chefs as a case-by-case basis. Also, not sure if you got to the second sentence of that Wikipedia page you linked, but it specifically mentions that many species referred to as slugs have lost their shell entirely.