600 Xbox + 360 games, all One games.wrong
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.