I was playing portal before it was available for linux (it was 2008 and I don't think there was even a mac client for steam yet). It ran faster on mac under wine than it did on windows, but I did notice wine was downgrading the graphics a bit (probably to DX8).

It's possible they were playing L4D under wine before the "native" linux client was released.

(the native client still uses a DX -> GL translator for shaders IIRC. Valve open-sourced it.)