All of google's backend code is written on Linux desktops. OSX is common for laptops, but real code isn't permitted to touch their drives.
Do you mean one is not allowed to check out the code on OSX and sync to the dev/linux box?
I still bet that the majority of engineers at Google use OS X on their primary laptops (and they still wouldn't be allowed to put the code on their laptops, even if the laptops were Linux).