Maybe they just have a contract saying they have the rights to publish the game on PC and that opened the door for it to be included on Gamepass. I doubt that Sony is happy with it but also no mad enough to do anything against 505, maybe 505 just had nothing to lose by taking the deal if they were offered good money.
Then third-party devs are left hanging if MS strategy of expanding Gamepass is successful. Good luck trying to convince people that got used to getting your games "for free" to pay $60 or $70 and good luck selling games on Xbox when it's entirely marketed around gamepass.
Then that would have been amended on Sony's end if they had an issue with it. 505 are in no position to fight on this. They're in a privilaged position