I think you're discounting just how much they've invested and continue to invest in Proton/WINE
I'm not really sure I am... Do we have some actual numbers into how much money they've sunk in linux?
Gaming on linux is a huge community effort, whether it's wine, dxvk, vkd3d, mesa, linux itself... and plenty of smaller projects like lutris, bottles, UMU... And all this spans literal decades, far before valve ever got involved.