First discovered by Windows Central, Microsoft's session report for the Game Developers Conference 2019 includes a breakdown of their intended topic. In the description, they have a session titled "Xbox Live: Growing & Engaging Your Gaming Community Across iOS, Android, Switch, Xbox, and PC"
"Get a first look at the SDK to enable game developers to connect players between iOS, Android, and Switch in addition to Xbox and any game in the Microsoft Store on Windows PCs."
To be clear, Xbox isn't launching an app or trying to sell games on the Nintendo Switch or mobile devices. They're making tools that make it easier to integrate Xbox Live features into a game. Microsoft appears to be promoting a new software development kit (SDK) that will integrate the Xbox Live platform into their games, regardless of platform. A perfect example of this is Minecraft: Switch Edition, which already has Xbox Live integration and crossplay.
This is only the latest in open-minded behaviors by Microsoft as of late. The company has been growing increasingly chill with working together in the gaming industry. Now only if Sony would play this nicely.