It's not optimism.
The first 1.3 million copies of the Witcher 3 that sold on PC, over half were on GOG, a platform that is strongly preferred by a huge number of cRPG enthusiasts.
That's a silly example. Witcher and GOG are both owned by CDP.
But there's probably a degree of overlap between PC users who bought Witcher 3 in the first month and PC users who will buy Pillars of Eternity: Deadfire in the first month.
I'm reasonably sure that RPGs are a genre that performs well on GOG, especially without a strong community or multiplayer element.
Over time, Steam sales from casual buyers will cause the Steam numbers to dwarf GOG sales, but the early, high expense sales should be keeping reasonable pace.
This is a genre that GOG went to painstaking efforts to market on their platform.