For Steam Early Access backers who have purchased the physical collector's edition, I expect the process to be this:
1) Receive physical CE.
2) Open physical CE.
3) Observe useless physical CE game code that can not be added to Steam because you already own the game on Steam.
4) Apply physical CE DLC code to Steam account to upgrade SEA copy of game with DLC goodies.
My problem with your comments is that we may not receive the physical Kickstarter edition of DoS before or on Monday so we won't have the game code to apply on Steam and would have to needlessly wait.