The port was done by another developer, Feral. As far as I know they used a tool to port the Direct3D stuff to OpenGL. But nowadays DXVK does the job much better.
But by now the support contract has ran out. So when the Windows version improved the Linux version just stagnated.
Interesting to see. The latest Tomb Raiders also have Linux native port, like Shadow of the Tomb Raider: Definitive Edition. Maybe they neglected it back then.
The port was done by another developer, Feral. As far as I know they used a tool to port the Direct3D stuff to OpenGL. But nowadays DXVK does the job much better.
But by now the support contract has ran out. So when the Windows version improved the Linux version just stagnated.