The game is available on two generations of Microsoft and Sony consoles, and it can also be played on PC. Sony is likely testing features, and Outer Wilds was marked by accident, but gamer HelpfulApple22 managed to take screenshots.
Right now, it is impossible to say for sure which games will be labeled as PS Plus Exclusive, but it is quite possible Sony wants to highlight adventure games that debut via the subscription, are available exclusively on PlayStation Plus on launch day, and are not on Xbox Game Pass.
Sony does not have a large number of games that have been offered through the subscription on their day of release. Adding them to PS Plus at launch could draw more gamers’ attention to the service.