Currently you can't buy game in physical form, but if Paradox decides to sell physical copies I would guess that they will be similar to those physical copies that backers get, which are drm free and have installer that is independent of any digital distribution service. But I can't say anything for sure.
In the update, they stated the retail version would be tied to Steam.
Well, shame . Sure the game on GOG heap or pile it all. I have a rule that does not buy games on fixed -dependent activation of digital services ( Steam , Origin ) because these games as you can not even install without connecting to network . Well at least Upaya does such problems. Installing of- line and activate as suits you . I miss the times when the game was not combined with the need to pour all this digital crap .