I don't see how that would apply in this situation. Perhaps it boosts sales AFTER it appears on Gamepass but I can't see any reason why it would have a positive effect if that information leaks long before it actually hits the service. There is no way this can be anything but a negative to sales during that gap.
Regardless, Spencer revealing this ahead of time was a blunder, which I'm going to assume was an honest mistake. You don't reveal stuff like that without the developer's permission. He might have literally just taken money from their pockets.
Maybe. And in my case it definitely would have stopped me from buying Control. Still, many people love to own things. Netflix (and Now Disney+) get content so fast but there's always people who ignore that and still buy Avengers Endgame or The Last Jedi. I usually only play games I'm curious about on Gamepass. If it's something I want to own, I buy it. I've even pirated movies only to turn right around and buy them because they were so good.
I don't see where the maybe part comes into play. The issue here is that there will be a period of time where the game is NOT on Gamepass but people know that buying it would be a waste of money if they have a Gamepass subscription. Is it possible that some of those people will buy the game because they played it on Gamepass? Sure, but that would have happened regardless. That eventuality did not need a long gap between the announcement (via Spencer) and the game actually appearing on the service in order to happen.
That's why the optimal situation--and one the developers probably wanted--was for the announcement to occur right before the game debuted on Gamepass.
This early announcement can only be a negative for the developers. How much of a negative is up for debate, of course, but I don't see any way it can benefit them. The only party it can benefit is Microsoft, as it might possibly push someone away from buying Control and into subscribing to Gamepass instead.
The developers would have a right to be unhappy. I know I would be.