I got my paper letter too, scheduled for morning Jan 21.
I'm told there are several interviews concurrently. My off-the-cuff guess from looking at the acceptance rate for WA students and a bit of reasoning is that they probably do take around half or slightly more of in-state interviewees. I further suspect that they have a good idea of who they are going to take prior to the interviews and unless you come off like a total dunce the interview probably don't grossly affect your chances.
From the timelines for the interviews, I highly doubt they have sent out any out-of-state interview requests yet so I wouldn't worry for folks in that box.
On the other hand, if I were an in-state student who hadn't been notified of an interview yet I'd probably be moderately concerned, though it is entirely possible they'll do another round in coming weeks -- Jan 20 is a long way away still.
I don't know why so many people have confusion with the Tier classifications, as it is actually extremely clear on WSU's criteria website. How did y'all get through your stat & prob prerequisite without learning about the distinction between logic and/or?