"The team developed the Heutchy Method (after the engineer who created it) which leverages the power of Xbox One X to let the Xbox 360 emulator present the very best version of the game possible with the existing assets—all without touching the game code. The same method improves the graphics for Original Xbox games on Xbox One and Xbox One S."
I know I'm posting a lot but I think this is the last bit for now. This could mean that a game had to be BC back on the 360 to work now. They are likely going to go beyond this limitation and open up work on bring over other games, but at the very least I think this means the old list means all of those games are possible, barring licensing issues. All 13 launch games are on the list. Looking at the entire list about of third have some kind of widescreen support.
Finally, here are the games at launch that have a 16:9 aspect ratio option:
Bloodrayne II, Grabbed by the Ghoulies, Ninja Gaiden: Black