Just because a language knows what you mean to do and can give a specific error messagr doesn't mean that, in general, and specifically, the developers want to

Just because a language knows what you mean to do and can give a specific error messagr doesn't mean that, in general, and specifically, the developers want to commit to supporting && as valid syntax forever. Perhaps the language creators have plans for future syntax that would not allow what you want. Even if for now the language evidently knows what you mean from the error message. In this specific case, the developers of fish apparently decided && was good to add.