At the risk of being pedantic, that's not exactly what the principle says. It's claim is that a cryptosystem should be secure even if everything about the system except the private key is public knowledge. It doesn't require that the system be public, only that the security of a non-public system shouldn't rely on it's non-public nature. A closed source cryptosystem designed to still be secure even if someone discovers how it works satisfies the principle just fine.
