If they're using Linux, they can find and patch the vulnerability immediately. They could even have a team hunting through the source code looking for vulnerabilities to patch.

On Windows, they would have to report the vulnerability to Microsoft, and hope that it's patched soon. It probably would be, but that's still an extra step - and they wouldn't be able to locate it with reference to the code. Also, as another person mentioned, china has access to the Windows source code; Google doesn't. This evens the playing field.