Fish and shellfish are both recognised and problematic allergens which are required to be notified as allergens when serving food by EU law, so not quite harmless.

Also; allergens and special diets is tricky, especially when they don’t correlate 1-1 with the special meal options. E.g. strawberries, soy and sulphites.
Sometimes the type of preparation also makes a big difference, e.g when it’s a cross allergy where your mouth essentially gets “confused” and mistakes something for an allergen. Friend of mine can’t eat certain foods raw, but can do cooked/processed because of this (and yes, that one is EpiPen level).

Point being that when a special meal is sold as a certain type, eg vegetarian, it really should be vegetarian.

Things like this are why those pre-select meals (as in the actual dishes) with a detailed ingredients list are so important.
The detail level like when you buy a product in a store is specifically important, as many things are not always made the same way. When ordering a beef meal, many people don’t realise that fish sauce may have been used for instance. Or all the different ways in which soups and sauces can be made.