Who said Gamepass is contributed more than advertisment? As you said nobody knows without working at one of the companies. But the fact remains PS+, GP, ... can contribute to sales due to affecting how much exposure a game is getting.It increases exposure but then so does ads and many other things that the platform holder is doing, so why pick Game Pass as the one that likely contributed more to exposure than any of those other things?
Doesn't mean subscription is the main driving force. But it also doesn't mean it's not the main driving force. This will depend on the game and the developer. Not everyone has the budget to reach millions and millions of people with ads. Some need to have partners like PS and Xbox, who happily pay a shitton of money (to add the game to the service) and provide exposure with their services for the game.
Take Rocket league for example, who's dev said they didn't think they could reach as many people without the PS+ deal and did say "it was the best decision ever", due to 6 million PS+ users talking about the game and adding to the exposure.