Maybe it wouldn't have had even a million players if it was purchase only. That's the way to look at it.
Potentially they got paid more than that, so they are win, win?
So they could have sold 1 million, made less money, less players and slipped into obscurity.
With game pass they got more money, more players talking about it, making threads on forums, posting on social media, streaming the game, more viewers, more marketing, boosting sales on ps5 and PC to what they would have....then...it also allowed them more of a tail on marketing becuase their teams can talk more about milestones hit in player count...refreshing the game in people's minds...more people notice its on game pass....decide to fire it up becuase they hadn't tried it....but 5 million players must mean its worth checking out, right?...more players....more posts....more streams...more sales....rinse and repeat...
Pretty cool, right?