This "Xbox" exclusive was once a Google Stadia project. Because of affairs in recent years with Google Stadia's studios and support studios, Kojima either had to scrap the project or go to someone else who had Cloud technology... Only other logical player was Microsoft....
If you look at Sony's official partner studios, Kojima Productions is still listed. Kojima is still working with Playstation (also keep in mind Kojima Productions and Guerrilla Games have very close ties to eachother)..
Kojima would be allowed to make other games for other platforms and I'm willing to bet the next game after this Cloud project is either a PlayStation game or it is multi-plat.
Kojima has stated he wanted to do small scale projects alongside big budget titles. I sense the MS title is something on the smaller side, hence why I said that I hope his next big project utilizes Decima.
"I sense the MS title is something on the smaller side"
Kojima is working with Xbox to build a game based around cloud technology. Think of it like the Strand concept in Death Stranding, but he wants to further explore what's possible. That isn't going to be a small scale game.
Here's hoping Kojima's next big project utilizes the Decima Engine again.