Exactely. They can't just add Xinput support alone, or at least I don't think they can.
They should just change the support from DX8 to DX9. Is this possible? In theory it is, of course.
But practically?

I have no clue how much it's a matter of bureaucracy, paying license and updating a couple of easy fields, or if it would require a major rehaul of the game code. Eitherway, I'm afraid to say it's unlikely to happen.
They could though expand the in-game pad configurator to add further options allowing us to circumvein the issue even mantaining support to DX8, while still using Dinput.
Like treating Zaxis+ and Zaxis- like two digital buttons instead than analog.
I'm pretty confident this can be easily coded on their side inside the Gamepad configurator without some major re-coding work.