It's just the way 'things' operate. Think of a prosecutor and a defense attorney. Sure, there is discovery. But neither reveals their plans. Same with competitive business. It's not a good/evil thing, it's a success thing.

I think dark/impenetrable means obscure and secure.

And, dark/night is easily penetrated by light, so to run with that metaphor:

"let your plans be secret and secure, but transparent for those who have the light"