But it IS backwards compatible in the way you are describing. You can play a dx9 game on windows 11. So it is backwards compatible. What you cannot do (usually) is force a game built with dx9 features to use dx11/12 features. If the game wasn’t built with new API features (because it released before those features even existed) then you cannot expect it to be able to just “be dx12” all of a sudden.
But it IS backwards compatible in the way you are describing. You can play a dx9 game on windows 11. So it is backwards compatible. What you cannot do (usually) is force a game built with dx9 features to use dx11/12 features. If the game wasn’t built with new API features (because it released before those features even existed) then you cannot expect it to be able to just “be dx12” all of a sudden.