There is nothing there that labels them as an bonus that will only ever be available to Kickstarter backers. Only that it is being given to Kickstarter backers as a reward. It could be given as a reward for other things as well (promotional, pre-order bonus, CE item, etc).
And, by the way, I've had more than enough "exclusive" pre-order and CE bonuses eventually become available to everyone to know that the term "exclusive" usually only applies to that moment. i.e. "this item is currently exclusive to this group at this particular time, but will be available to everyone at a later point.". Developers rarely hold to the exclusive promise the way people expect (not that it matters here, as the term exclusive was never even used).
They could but I doubt it for the simple reason this is a more touchy situation than usual....we're talking about backers who pledged on faith. Without them Original Sin would be a much lesser game. I'm not overly concerned about this myself but I can see how some people would be upset. And this isn't a publisher looking to nickel and dime every bit of DLC imaginable like all the preorder bonuses that later went on sale. But I'm sure modders will make them more widely available so I don't really get the concern here.
