Back in May 2018, we saw the Game Rating and Administration Committee of Korea rate Sunset Overdrive for PC. While ratings board listings are not official game announcements, for a game to get rated the publisher has to pay a lot of money, provide evidence the game exists, and generally isn't able to do this until the game is relatively close to release. It's about as close to an official confirmation as you can get without the creators officially announcing the game.
Now, a second ratings board has rated a PC port of Sunset Overdrive, further confirming the imminent existence of the game.
The ESRB rating that popped up today doesn't reveal anything new; it just reaffirms that this release is on the way. With all of Microsoft's modern exclusive titles coming to both Xbox One and PC on the same day, it makes sense for older exclusives in their back catalogue to get PC support too. The question now is when this will get announced, rather than if.