Quite a lot to take away from that tweet.
Indeed, there is quite alot you can take from that:
-MS has either completed the design of the system, or at least created a working prototype of it. I doubt he would taken a bulky "PC in a giant case" devkit home with him, which likely means that he has a working console form factor system.
-The operating system is working and already allows connection with the XB1 ecosystem.
-All of the Xbox One games he plays have working backwards compatibility on Scarlett.
-He mentions an Elite controller, rather than saying that he has the new Scarlett controller. This either means that the Scarlett controller (and Scarlett Elite controller) aren't done yet, or that they are bundling the Elite V2 with Scarlett instead of designing a new controller. Most likely the former, the Elite V2 controller costs too much to bundle with a system that is likely aiming for $500 or less, and it would also lack features such as the haptic feedback of the PS5 controller, which MS has surely heard about by now and will want to have similar tech in their Scarlett controller.