wrongI'm aware.Correction: To date, we’ve spent over 100K hours testing your favorite games from Xbox One, including existing backward compatible 360 & OG Xbox games, on Xbox Series X. While we are still in the process of validating, we can confirm thousands of games will be playable at launch.
— Xbox (@Xbox) March 20, 2020
when asked directly they refused to say yes,. they have the same answer as ps5's clarifying statement of "thousands" of games.
series x BC is in the same boat as ps5. most will work without changes,.. some will need changes. testing is required to formally state compatibility.
They're never going to promise every One game. There's almost certainly going to be some random case that doesn't work as intended.
And they tend to get sued over things like that.