RReady creates an opengl context using the window handle passed into rredline.
It's the same window and it works because the window hasn't been made directx compatible.
Actual rredline has no issue with the surface being directx compatible, but rready does. In fact that's why games like interstate 76, myth tfl and MechWarrior 2 series need dxwnd or some other directdraw wrapper. Thats also why MW2 can't switch from opengl to directx and vv.
Windows only allows a surface to be directx or opengl compatible. A similar restriction exists for vulkan, a whole new type of surface incompatibility.
Developer of RReady - Rendition Verité Wrapper.
https://www.youtube.com/@sharangadayananda