And similarly the games themselves are the same across platforms, apart from their end of the WGL or GLX or whatever bindings. So anything in the driver that says `if (!strcmp(name, "Portal 2"))` is going to be equally applicable regardless of whether it's a Portal 2 compiled for Windows, Mac, or Linux.
It's certainly a weakness of the all-free-software subset of the Linux platform (at least on Nvidia or AMD), but that's not the only option.