Looks like they’re pushing Mixer more and more from within Windows. I think if Microsoft continues to very closely integrate Mixer with the OS, it will prove to be a significant market advantage over Twitch, et al.
Granted, the update was only this:
Improved gaming features. You and up to 3 friends can combine streams into a single view using Mixer. Plus, you can easily switch on Game Mode right from the Game Bar for even better performance.
But what it shows is that they’re investing in the platform and that the Windows team is on board with it.