They probably use some outdated translation layer that converts DX9 calls to OpenGL ones so that they didn't have to maintain two renderers or use a higher leve

They probably use some outdated translation layer that converts DX9 calls to OpenGL ones so that they didn't have to maintain two renderers or use a higher level 3rd-party engine that abstracts over those calls. Newer games using Steam's Proton, like Sekiro, look and perform nearly equivalently to their Windows DX11 counterparts. I know Proton isn't exactly an apples-to-apples comparison, but it's an example off the top of my head. I guess CS:GO is another more apt one.