As much as I've figured, and it helps me to think of it like this is tha it's not so much super-effective to every type (else it'd do 4x damage to all dual-types) as much as a Shadow attack is super-effective to a non-Shadow, as if there were an extra sort of variable in the game as to whether a Pokemon was Shadow or not (which it probably is), and the supereffectiveness is determined by that, and not necessarily by type.