You are almost correct. Here are the details how it works:

This Premium Edition is a so-called "sub package" that is just a "container". The actual items you can own are inside of it. Open the Steam Store page: http://store.steampowered.com/sub/52852/ There you find a listing of the items included:
Basegame: http://store.steampowered.com/app/313690/ (note that this is "app", not "sub")
Season Pass: http://store.steampowered.com/app/330860/ (also "app", not "sub")

As far as I know, currently SG simply checks that do you have item in your Steam Library that matches http://store.steampowered.com/sub/52852/

You cannot own "sub" items, you can only own "app" items. Since nobody has "sub" items in their Steam library, everybody are able to enter these package gibs. To fix this issue SG would need to make their script better. That script should check the ownership status of the "app" items that are inside the "sub", not the "sub" itself.

Why? xD

Alright, entry withdrawn. Thanks for the quick answer!